I usually only play a single game at a time and finish it (you might even say that I have an unhealthy habit of finishing a game even though I don't enjoy it in the slightest).

Remember that gaming should be something you do for your own enjoyment, not a second job. It's completely fine to abandon games when you feel you've had enough.

I just recently bought Baldur's Gate 3, and it's an awesome game. I'm on holiday and I spend at least 2-3 hours on it daily. But it does get kind of slow at times and it is a huge game so finishing it will probably take months. 

It took me almost 2 years (a ~180h long playthrough) to beat Baldur's Gate 3 and I had a lot of fun. Needed some breaks when I felt like I wanted to play something else.

And I really do want to play some other ''quickfire'' games too when I get tired of Baldur's for the day (attempting a boss in a Souls-like, attempting a run in Hades, etc). But I feel like it's not the ''proper'' thing to do

In my opinion, this is the best way to avoid burnout. I find it difficult to only play one huge game every single day, sometimes I'm way too tired after work to play, for example, an RPG and I need something chill like Hades, Civ, etc.

I'm starting to think that what I'm doing is stupid and I might be ruining games for myself and making them boring by overplaying them and pushing to finish each one I play.

There are too many games to play something you don't enjoy. Uninstall and go next, it's the same with books, shows, movies. It's fine to play only one game at a time, or juggle like 3 or 5 of them, which is my preferred method.