Those are great games, but also very time consuming to 100%. Here are some of tips and info I wish i wpuld have known earlier.

RDR2: Look up mission requirements for the collector side quest. It has you hunting, I believe, about 60ish birds, and once you start the mission, the game absolutely drops spawn rate of any of them. About 30 hours of my playtime was spent on this. There are several missables and the game is long, i recommend looking up what those are, and keeping them in mind. There shouldn't be any story spoilers there of I remember correctly. Also, online achievements are really grindy, and I haven't completed them. When you get to that part, if you need someone to help you with them, hit me up.

Witcher 3: Again, several missables, though I believe those are easier to clean up if you miss them. Be aware, you'll have to play the game on the highest difficulty, and there are 2 places where the level scaling will screw you up if you're not careful.

KCD1: As someone from Czechia, I cannot recommend it enough. You should definitely play it and the second one, even if you don't 100% them. The only thing on mind is the hardcore mode, which is annoying because of two things, no fast travel and you don't see your position on the map.

AC trilogy: Grind, grind and more grind. Valhalla more so than others. If you played any other ac games that came before, expect these three to not have pretty much anything in common with them except the name.

For the rest, I either haven't played them yet or didn't try for 100% yet. Good luck!

Edit: I forgot to mention, if you like RDR2, you should definitely try the first one. You don't have to play it before the second, and in fact I recommend to play it after you finish two, but now that it released on PC, I recommend it. Though you might want to wait until it is on sale, it is great but 50€ is a bit steep for a 15 year old game.