How do I max a character once I've completed all the maps?

[hankova14]

Playing the same maps again does not give evolution materials.

[Waste_of_skin]

There are several ways to get those resources. Quoting the Wiki [bloons.fandom.com] :

Random character resources can be obtained from the following:

Earning a new medal on a map
Earning all medals of a certain difficulty in an Adventure
Completing a game on a map under the influence of the Bloon Beacon
Earning achievements
Completing daily quests
Protecting a box at the end of the track during the Fireworks Fun!, Hall-ooo-ween, and Holly Jolly Secrets Events (not Joshua and Margaret Investigations)

So, if you've completed all maps on all difficulties, it's mainly Bloon Beacon maps and the daily quests.