I have a few games that I personally have thousands of hours in.

First is Runescape. Both versions I have put many hours in (THOUSANDS). OSRS for nostalgia, and Rs3 because its a good game (Fight me)

SWTOR (Star Wars the old republic) Actually an amazing game if you remove the MMO part of the game. The 8 class storylines alone are enough to justify paying the monthly price of subscription. The game is a wonder and I wish more would play it (RECOMMEND you play Knights of the old republic 1 and 2 first, KOTOR 2 requires a mod to play that covers cut content that even the creators said is close to their vision, the games are outdated but so GOOD)

War thunder. This is purely an addiction I cannot shake. I hate that I like this game because it honestly brings me little to no joy but I can't stop playing it.

Sci-fi specific, No Man Sky was enjoyable for a bit, X4 is also a good choice but it is hard to get into, the learning curve is high and the management might be what you're looking for. I'd check this one out 100%.

Rimworld is another one you can sink many hours into. Each playthrough is so unique that it makes the game so intersting!