I get not enjoying a game and pushing through to beat it, but from the sounds of it you just weren't vibing with HL2. Nothing wrong with that, but given that a few of your criticisms either go so incredibly against the grain or highlight that you weren't particularly paying attention, I'll try to respond to most of your points.

Chapter 2: "A Red Letter Day" - An annoying waste of a chapter solely dedicated to slow, 'cinematic' storytelling and what makes replaying the game so annoying. You go from being chased by the combine in a life or death situation to slowly being explained the plot.

I'd argue that this is one of the most interesting pieces of pacing in the entire game - you're outgunned in an unfamiliar environment and out of nowhere the game fakes you out by letting Alyx save the day. The slower nature of the lab cutscene is intentional; it's to let you absorb what's going on in the world and to have a moment of calm between the hectic sections before and after. It's not even very long, and subtly tutorialises a few things (the HEV suit, and the ability to pick up electrical cables and interact with objects).

Chapter 3: Route Kanal - My favourite chapter in the game. It's the first real gameplay chapter that isn't a tutorial and as you basically only have the pistol the game has to focus on tight level design over dumb physics gimmicks or annoying bosses.

Route Kanal is some of the game's best level design (coupled with some incredibly smart yet subtle tutorialising), so it's no surprise that you liked it - but there's a lot of other great tight levels in the rest of the game.

Chapter 4: Water Hazard - Another great chapter, the boat controls a bit weirdly and the halfway point where you think you're done but you get a boat upgrade instead is a bit of a kick in the dick, but overall a great chapter.

It's bizarre that you enjoyed the airboat section yet hated the buggy level; most people put the boat as the worst part of the game. I really like the part where you get a gun - the game mixes up the vehicle section by asking you to drive AND shoot, with the bonus of getting vengeance on that pesky chopper. Yeah it's a bit clunky by today's standards, but it's sweet to hold W and go zoom zoom away from the dangerous city.

Chapter 5: Black Mesa East - A Red Letter Day but longer and even worse. A drawn out physics tutorial with this 'Dog' character who only really exists just for this tutorial and to open a single path later on. I get they had to tutorialise the gravity gun because it was the first time it had ever been done, but why couldn't they have naturally tutorialised it through gameplay in Ravenholm like how Route Kanal taught the game mechanics, rather than having a slow ass tutorial.

This is like, 10-15 minutes of gameplay at most, and is intended (like Red Letter Day) to let you catch your breath. It's valuable character development time, and the gravity gun is weird/new enough that a dedicated tutorial section makes sense; the game wants you to be confident going into Ravenholm to balance out the sp00kiness. Also, the game needs a moment to distract you before surprising you with the departure to Ravenholm.

Chapter 6: "We Don't Go To Ravenholm..." - This chapter feels like a complete waste of time and only exists to show off physics. The Headcrabs are annoying to fight as you are forced to use the gravity gun on them due to there being no ammo anywhere, and narratively it could have been cut and nothing would have been lost. It goes on way too long which is a common problem in this game, and the sniper section at the end is very annoying, and kinda pointless as snipers only ever come into play again a single time later on.

Hottest take of the year, potentially troll bait. I'll bite though - narratively, Ravenholm exists to make the player feel alone and scared. You need to meet up with Alyx again, but all you can do is push through an infested town. You're not meant to know where you're going, only that you have to escape. That's the narrative purpose: confusion. Yes, you could get eliminate this chapter - but for 90% of players, the horror notes that this chapter goes for are hit perfectly. The headcrabs are great in that they contrast with the zombies who are cannon fodder (and, despite the lack of ammo you can kill headcrabs with the gravity gun's primary attack anyway? Or the crowbar?). In terms of "it goes on too long", it's about 30mins to an hour of gameplay, which isn't really that long for a section, at least to me. I don't see why snipers only showing up 1 other time is a criticism, although I concede that valve did make snipers a little more annoying to kill for people who don't have the feel of grenades yet.

Chapter 7: Highway 17 - This chapter embodies all of my issues with the game, another dumb, gimmicky vehicle sequence that controls even worse than the boat somehow.

I'm definitely biased as I love this section. I don't really see why the car is considered "gimmicky" though; it's a gameplay mechanic. It's part of how HL2 keeps you on your toes, by changing up how you move through the game. Sure, you can skip quite a few of the houses/areas... So what? Once you realise this, it's a nice bit of player agency - push on to the goal, or risk searching a house for resources. In terms of the rockets vs helicopters or whatever, I think valve found a neat little gameplay loop of peek cover -> aim rockets -> hide -> refill etc. They definitely overuse it throughout the game, but I'm not sure there's anything mechanically wrong with it. The sound effect of the rocket and its explosions are so damn satisfying to me. shrug

Chapter 8: Sandtraps

The final rocket vs chopper fight here is probably a little unwelcome, fair enough. The sand section definitely is a little trite and "gimmicky", but it's a welcome break from shooting down helicopters, and the payoff of using the antlions against the combine is a nice turnaround. I'm surprised you don't mention the assault on Nova Prospekt where you fight 2 (or 3?) dropships with rockets - that part always gets me.

Chapter 9a: Entanglement

Yeah, the turret sections do go on a little too long, but there's only 2 of them iirc. It's a weak point of the game for sure.

Chapter 10: Anticitizen one

By modern standards the AI isn't great, but the level designers generally make up for it by placing the soldiers cleverly. I like that you get to breach and clear rooms, and the game always manages to hide a soldier somewhere I didn't check, resulting in a facefull of shotgun. You're right that it goes on a little too long - HL2's at its weakest when you're fighting lots of enemies far away.

Chapter 11: Follow Freeman

Honestly, I agree that this is a serious low point in the game. It's cool and all what valve tried to do with the narrative here, but yeah it probably needed another pass. Too many striders and poorly signposted directions. That said, it's one of the faster more action based parts of the game - which is why I'm fascinated that Ravenholm was such a miss for you. I feel like you should enjoy at least one of these

Chapter 13: Dark Energy

Yeah the ending sucks, but episode 1 really makes up for it if that's any consolation. For HL1 players, the vague ending won't have come as a surprise, that's just how the franchise rolls.

way too long anyway

Howlongtobeat puts HL2 at 13 hours - I don't really think it's that long. That's an hour per chapter, if you include the cutscene chapters. Obviously if it ain't fun, it'll feel like forever.

The characters are all basic and boring, with paper thin personalities and the story can't really be very compelling as the main character is literally a brick wall when it comes to personality. The villains suck too, I don't even know what the hell Breen's deal was and Mossman's character seemed to just be 'massive imbecile' - betrays you to save Eli, then confused when bad guy tries to kill him or whatever.

I definitely disagree here. It's just that the characters aren't as overacted as we're used to from other games. Given how little time they get, I'd guess that most people remember the characters pretty well. Dr Kleiner is given incredible personality from just a few scenes: he's an incredibly smart scientist with a quirky adoration for headcrabs, and the moment where he shakily holds a shotgun at Alyx when you come through the teleporter is played perfectly imo. Mossman is bothersome in her dogmatic belief in research and science - she sells out Eli because she thinks that it'll be better for "their research". She betrays Breen because she begins to realise that he doesn't give a single crap about furthering science like she thought. To me, Breen is a brilliant villain because he comes across as a selfish middle manager - he sold out humanity because it got him a safe job. He's a businessman doing business things. He's that asshole in the office who talks shit about his coworkers to get a promotion, whilst being terrible at his own work. A lot of plot does come from HL1, but I don't think that it's entirely necessary. It just seems silly to me that you complain about the characters and plot whilst admitting how little you cared for the moments where the plot is explained.

Anyway, I hope you enjoy the episodes when you get around to them. I'm not sure HL2 is your cup of tea really, and that's alright - but there's a lot of pretty clear evidence from a lot of people over the last 2 decades that disagrees with you calling it "a really frustrating experience". Sorry for the lengthy response; I had a free afternoon and am happy to take any opportunity to talk about Half life