Agreed as well. I think you absolutely can top Thanos as a villain, but it makes more sense not to try. There was enough of a build up from Avengers 1 that it made it semi-personal for Tony, Cap, etc. Then he of course had the history with Gamora and Drax, and then killed Loki in front of Thor, snapped Hawkeye's family, etc. Having two movies to spend on it helped a ton as well.
It makes a ton of sense to split the cosmic and the earth-bound, because there's currently not much tying them together. What team ever had Shang Chi, Spider-Man, Captain Marvel, Star-Lord, Doctor Strange and Black Panther? None, really. They're about as dispersed as you could pick.
But you could easily have some team-ups and cross-overs among that group. For example:
Earth: Spider-Man, Black Panther, Shang Chi, Falcon, War Machine, Hulk, Scarlet Witch, Doctor Strange, Thor, Fantastic Four, Captain Marvel
Cosmic: Guardians of the Galaxy, Thor, Fantastic Four, Captain Marvel, Nova
THEN AGAIN... it really was that easy for Endgame to say "Who hasn't been to space?" and have literally every major character meet each other in the final battle. War Machine isn't traditionally a space character, for example, but he did it twice in Endgame. Silver Surfer comes to earth in the comics, Hulk went to space in Ragnarok, etc. There's enough logical crossover for Thor, FF, Captain Marvel, etc. for them to bring someone along for the ride.
Maybe the best way to look at it is by villains. Doctor Doom is predominantly earth-based, but he does venture into space from time to time. Magneto is predominantly earth-focused. Galactus is cosmic but comes to earth. Korvac is a GotG villain who comes to earth and fights the Avengers. The Kree and Skrulls are cosmic but frequently come to earth. The Shi'ar are cosmic but regularly encounter the X-Men. Annihilus is predominantly cosmic. Etc. etc.
Personally, I think the two live in their own realms, but you have enough Thors, Captain Marvels and Fantastic Fours to bridge the gap at any time. Spider-Man meets the Human Torch, Galactus comes to earth, suddenly Spider-Man is in space again. Things like that, if it ever makes logical sense. But I doubt we'll have another Endgame-style, "literally every character" kind of cross-over again. Before too long that will just be too much!