The time-since-death donation restriction approach sounds fairly nice.
To explore some alternatives:
Separate the wages minimum spawn money from bonuses earned. Be unable to donate money from the spawn wages. (Side effect: can't share legitimate money at start of round, as stands.) As a side note, it's kind of annoyed me that you keep none of your bonuses until you cross your wage threshold, and that lately I've been thinking, "I'm likely to die soon so I should quick/scramble to donate my petty cash as otherwise it'll (up to 1 x wage) go to waste and I'll start with the wage regardless."
Or, if spawn wages were doled out over the span of, say, 1-5 minutes (with the exception of first spawn) up to your wage cap (measured separate from or together with bonuses?). So, at start of round you have 20K, and as you buy or donate, your 20K wages from next spawn start flowing in, over time. You do 5K in damage, your wages stop when you have 25K... Perhaps you could keep a lower minimum, say 10K (with timer-delayed donation and further wages flowing in normally... though then you might get stuck in a minimum+flow loop, having spent your next life's (mech's?) wages on the previous life/mech -- maybe just pay wages over time up to wage cap, not tied to spawn? Or, tangentially, wages over time up to [wage] with minimum spawn cash the same, just to help keep bonuses?) Some formulation of this might address another annoyance/quasi-exploit: aero, bomb, crash, aero, bomb, crash. Or solo suicide charges, just trying to get in and do a little damage for rank, even though it hurts the team overall. Side effect: some variants make it harder to shake base-rape.