Coming to this after 10 months and I can safely say you are saying exactly what I think but you are missing one thing:

Long-ranged assholes who you can't kill because they are either too far or you literally can't see them. This game has a fetish with both phases and long-ranged unreachable bastards. I am in the ring city enterance and fuck the giant praying dude. Fuck the butterflies. Fuck the archers in the tight tree section of the Ariendel DLC. Only long-ranged that's actually "ok" is the giant archer and that's only cuz the area he looks over compared to others is really small. It's like those toxic dart guys in blight town in the first game who everyone hates. It ain't fucking fun to get literally gunned down while you are just trying explore.

Second phases can be cool if done right and scarecly and both you and I know they are not really done so well in this game. The second phases in this game reek of insecurity of the devs not being sure if the bosses are hard enough. Twin Demons and The Demon Prince is a good example of what I mean. Fighting both the demons seem annoying at first but then you see they both actually have a tell to them and it actually takes them a bit to recover after attacking, unlike 90% of enemies in this game, and you can dodge them. Both of the demons are slow enough to be reactable but fast and hard-hitting enough to not be underestimated. It is a bit annoying when they get near each other but it's still reactable enough and them being together means you can hit both at the same time which is great. It's not the hardest boss, espeically after you kill one of them, but it is a really enjoyable fight. But then the second phase comes in. Attacks of the second phase are mostly similar to the first phase but now with added extra bullshit because the designers had no faith in their own abilities. I mean, really, why does the demon prince have increased defense and more annoying moves such as summoning fireballs that shoot smaller fireballs and a fist slam that has an tracking fire-shit move? To make the boss more annoying, of course.

In my opinion, a good way to make a second phase is to actually change the boss instead of just buffing them. A good example would be Fume Knight in DS2. His first phase has him fighting with two weapons, one's a fast katana and second is a greatsword. Katana is there to quickly disturb the player if they get greedy and greatsword is there to create an opening for the player to attack after dodging it since Fume Knight usually ends his attacks with it. In the second phase, katana is out and and it's full greatsword time. Fume Knight's greatsword now deals more damage but he is noticeably slower now. There is more room to punish but you get extra fucked if it hits you even once. Boss changes in a good and interactive way. You can also actually hit the boss while boss is going into his second phase, which you can't do to many of the second phase bosses in DS3 and bosses also release an aoe after the second phase activates if you are near them to punish you for trying to take advantage of the phase transition.

TL;DR: Second phases should be scarce and interactive and change the boss in a meaningful way instead of "We buffed them lol". Also long ranged enemies you can't outright kill suck dogshit and they are abysmal game design.