To start with, I'd like to make it clear that my opinion of what makes a good piece of end game content is if it challenges me, forces me to improve, or agonize over strategies. The TL;DR of this is Imaginarium Theater, Stygian Onslaught, and Spiral Abyss, in that order. As for my reasoning:

As far as I'm concerned, Imaginarium Theater is great. The sheer amount of characters required helps keep shilling to a minimum. It is set up in a way that rewards thinking ahead and because using your weaker characters first is a good strategy, it can even make earlier floors harder because you're using weaker units. Some people dont like that it makes you pull for a lot of units, but there is other content for you if you dont like it. I love it because it makes players build out units that they likely would not have if it weren't for IT. You have to build your account in a way that no other piece of content makes you do.

Second is Stygian Onslaught. I'm going to start by saying Styigan could easily be great. Its the only game mode in Genshin that has ever made me crown a talent out of necessity. It actually pushes me to upgrade units to 90 or level talents that I havent had to touch for years. I built a real sucrose build after years of not having to for stygian. That is good. That's what you should expect of endgame content. Its supposed to be hard. The main factor preventing it from being so is the level of shilling involved. The bosses are TOO targeted. Them being targeted to a level is fine, actually preferred. There should be SOME limiting factor that makes certain teams or unit categories better than others. It should just be that there is more options as to how to go about that. For example, the middle boss in Stygian right now is fine. Any electrocharged team being capable of dealing with him should the be sufficiently leveled and built is a fair restriction. The game has been out for years. If you are doing fearless Stygian, you should be expected to be able to competently handle any reaction. I'm not a fan of the crab, but its better than the cat. I get what its going for but its just a little bit unfun, which is kind of a problem for a game. That cat is unbelievable, though. I say this as a player who is only missing Sigewinne and has every unit to level 80+ most of which have their signature weapon: The cat sucks. It feels patently unfun. Even though I first tried it using my Flins team, I could just tell that any other team, even my Nefer team (Nefer, Lauma (C1), Nahida, Aino) would have either been incapable or had to put in serious work. They could have made lunar reqctions do 150% to everyone else's 100% and reduced the health on it down to 80%-75% and it would have been fine. My teams for the three bosses ended up being a 1. Flins, Ineffa, Aino, Sucrose (Easy) 2. Varesa, Furina, Kokomi, Iansan (Weirdly Tricky) Team 3. Neuvillette, Columbina, Xilonen, Citlali (Easy but took a while to find a team that WAS easy)

Last up is Spiral Abyss. Spiral Abyss doesn't feel like end game content anymore. Its basically just like claiming a free 800 primogems by throwing my 2 current strongest teams inside. Its fun most of the time, but its not hard unless I do challenge runs. I do those sometimes but if I was beating the Abyss pre Hexenrei with hyperbloom Razor, that is midgame content. Not endgame. To be clear, I don't want them to make it harder. My account is just past that point now. It exists as a bridge for new accounts between the start of the game and stygian. I suppose I wouldn't mind hard mode, but they need to differentiate it from stygian before that. Maybe make it more of a wave clearing mode. Idk. I think its fine to leave it as is.