Is it worth going back to older TES games after starting with Skyrim? I feel like I'll be annoyed by QOL issues and even more jank. Is Oblivion objectively fun or only something you can enjoy if you played it at the time?
If you only played Skyrim, Obvlivion would be not much of a problem. In terms of Gameplay it is almost the exact game. Only differences are of course graphics, some weird things like the dialogue mini game (which can be completely ignored... and should be ignored lol), etc... I also think Oblivion's main quest kinda sucks, Skyrim's was better (but also kinda sucked). Oblivion had the by far better guilds, tho. I especially liked the Dark Brotherhood and Thieves Guild in Oblivion. But it's been at least a decade since I played it for the last time, so take that with a grain of salt.
Morrowind will be kind of a culture shock for you. It is somewhere between the "Bethesda style games" and classic RPGs. It is still my favorite in the series, and the first TES I ever played.
In short, Morrowind has almost no voiced NPCs apart from a few important for the main story, so you will read a lot - if you even do the main story.
Combat is based on your skills and dice rolls like in a classical RPG, so prepare to be weak af in the beginning with your hits barely doing damage.
It also doesn't take you by the hand like Skyrim. You can easily overlook the main story.
Also no fast travel, you can only fast travel between certain towns by paying for a ride. But the world is actually so nicely designed and rather small that you won't miss it.
But it is still a great game which allows the player some freedoms Oblivion and Skyrim won't allow him.
For the first two games: I only played Daggerfall, which is kind of a broken masterpiece. I wouldn't recommend it for anyone who hasn't at least played Morrowind. Avoid that for now. It will give you the most opportunities for role playing of any TES title, but you really need to mod it so that it even works (Daggerfall Unity is kinda mandatory) and previous knowledge in TTRPGs or CRPGs is kinda mandatory to play it too.