GrapheneOS
FWIW, I can now also report that setting the network to only use LTE (without touching VoLTE or any other settings) results in substantially better battery usage, same as it seemed to be until a few months ago. There's no decline in call, text, or coverage quality that I notice.
I haven't tried my setup with a stock OS, but, assuming that this issue doesn't affect millions of stock users, it sure sounds to me like some networks (including T-Mobile in the US) have done something with their 5G service that hits grapheneos's battery usage particularly hard.
So I'll keep my fingers crossed that somebody in the community finds a fix for the 5G battery problem before the networks EOL their 4G service! Meantime, the LTE workaround seems to do the trick pretty well for me. Thanks to those here who suggested it!