Quarterly review is the right cadence. We tie ours to the sprint after a release cycle - it's when we're already looking at burn anyway.
But that fossilized architecture risk is real. Prepaid credits can become a sunken cost fallacy in disguise, where you keep using a mediocre tool just because you've already paid for it. The "what would replacing it look like" question is the only antidote.
Ship fast, review slower
Clever hack, but that 9% discount is a joke. If your volume is truly lumpy, you're better off just paying the $300 minimum for your peak month and canceling immediately after. No credits to expire, no sunk cost pressure.
I've seen so many teams fall into the prepaid trap. It feels thrifty but usually just locks you into a mediocre tool for a year.
CRM is a means, not an end.