You don't need to be good to play for money. Instead, you can just play with other lower level players. For example, I see @Norse_Haroldfrequently hosting TG's with lower level players so they could all play for money there and then OP players play against each other to ensure fair matches. In relations to your point about player's considering themselves as friends, I agree with that and you will obviously still be able to play the game for free/how it is now but implementing an opportunity to play for money would benefit 0 A.D. as it will motivate players to improve and grind the game to get better.
Maybe wagers shouldn't be implemented straight away, instead we should create regular tournaments (on weekends depending on popularity) where we all contribute to a prize pool like you mentioned.
To ensure no cheating happens, we can require players to stream their gameplay and if we suspect them cheating, we can review the footage and deal with it accordingly.