I personally never touched PES 2021, mainly because it was the first one they admitted was a Season Update (I mean, it was right there on the logo). Jokes aside, that was indeed the reason why I stuck with 2020: I didn't want to pay the full price for what was essentially an update package (and even if I did, I couldn't afford it anyway), plus I was already having fun with the Master League in 2020 as it was.
I only have to look for the right Option File packs to make my experience pleasant: my preferred combination of leagues is Germany for PEU, Mexico for PLA and Japan for PAS. And then I'd fill the "Other Clubs" with teams from countries that don't have leagues. I actually download these, because I don't have the time or the skills to make them from the ground up; I just go into Edit afterwards to fine-tune some things.
I just wish there was a mod out there or something that would allow me to edit the Pleasure Horn team in ML/BAL and move teams into that African Clubs block it inhabits so there would be more teams contesting the Club World Cup at the end of the year, and to make the transfer market more varied - I tried messing around like that in PES 2012 and it worked, so I have some hope it could work here.