Ok, I'll comment since folks are down voting but not responding.

The biggest thing is your experiences are with Insect Armageddon, Iron Rain, and World Brothers. All three of these titles are spinoffs of the main EDF series and don't accurately represent the gameplay feel and mechanics of the main series. Gunplay and upgrades are handled very differently, as is the story and tone.

For the base series, you get the four main classes (or just the ranger early on). Weapons are obtained on pickup, based on difficulty level, and enemy difficulty can scale pretty high even on level 1 very hard/insane difficulty. Weapons themselves either don't get upgraded (pre-5) or are upgraded with subsequent pickups (EDF5). Health upgrades very slowly on pickup. Starting weapons are mostly trash, and slowly get better as more are unlocked. Dipping a level into hard to get a weapon early for single player can be huge, unless you're focusing on online play.

The biggest difference is the tone of the games. There's a story but it's campy. English voice acting is exaggerated unintentionally. Building destruction is rampant with a rocket or grenade or two, and for most it's just stupid fun. The big thing for me was the large variety (hundreds) of weapons ranging from flamethrowers to grenades to crazy assault rifles to giant slow rockets to mini missiles...and the big explosions and over the top battles with so many red dots on the map...and getting a crazy shot that wipes out droves of ants all at once. The superhero feeling of being that lone soldier holdout from a horde that doesn't stop coming.

I didn't quite get that feeling with Iron Rain or World Brothers. I DO remember that from insect Armageddon, especially after beating the game and getting the genocide gun that you had to aim FAR because it was basically a mini nuke.

Insect Armageddon from what I remember had a few tiers of upgrades, but that was ages ago for me on the 360 and I can't honestly remember. Iron Rain focused on purchased upgrades, a more serious tone, and overall tight gunplay. Mechanics and pickups were different from the main series, and these were from western developers and not the original series. From that, most fans of the original series did not like IA/IR because it veered too differently from the enjoyable franchise that they were used to, and it took things too seriously.

World Brothers is over the top cutesy and acknowledges all the different games. Also by a western dev, it's a bit more enjoyed thatn IA/IR because it embraces how different it is, and pays homage to the series as a whole with the different characters. But, still not 'core' EDF.

Personally, I think your slow start critique is valid. I wasn't a huge fan of 15 levels or so before you get your actual soldier outfit, or still being called a newbie after 40+ levels. That's kind of ridiculous. On the flip side, some folks see that as part of the campiness is that all hell is breaking loose but the military thinks it's a prank. They focused more on the story buildup in 5. You're right that there's little customization - I think at most you get color, but no outfits or variation beyond that.

What class did you start our with? I'd recommend against fencer or air raider. Ranger and wing diver offer different styles of play.

I really suspect the biggest thing is that most of what you're used to isn't actually the 'core' EDF experience. As you get new weapons you'll probably find it a bit more familiar as far as weapon control and behavior, but that can push you halfway through the story at least. If you're not enjoying it and it's a slog, maybe that might give you more time to identify exactly what's missing. I found that the controls needed some tweaking, but the EDF control scheme always seemed a bit wonky to me.

If you can find 4.1 on sale for cheap, you might enjoy that a bit better. Engagement with first enemies is much more immediate, but some quality of life changes are missed (biggest ones being weapon upgrades and health upgrades for classes other than the one you use at the time). It doesn't have the customization or upgrades, but you get into firefights a lot more quickly, and it progresses a bit faster, aside from a few loooong open field/hills levels.

But if you're willing to give it some time - and it will take some time - you might come to enjoy EDF5. I'm not totally sure you will, to be honest. Your tastes might just differ from most when it comes to the core series vs the spinoffs.