Actually, I am surprised that stake offers something like deposit bonuses now, I never heard of that. Thought this might be a scam but since you say the bonus money was added to your account I guess it's not.

Anyway, I had the same problem with being limited on sports. This limit is mostly just for 1 sport, the one where you obviously have been successful, otherwise you wouldn't have gotten the limit.
I mean you could just run the required wager through the casino at like super high winning % bets, that would be a solution. Dice 1.01 odds for example. Sure you would have some calculated losses due to house edge but it's better than nothing. Or, as I said, you try to bet on different sports and check if the limit also applies there, normally it shouldn't.

Hopefully you can figure something out! Good luck!