As I understand you have ~same battery time on underclocked CM5 as on CM4
@Rex is using it this way and seems happy: Bookworm 6.12.y for the uConsole and DevTerm - #163 by Rex
there are still some issues with CM5 (software and extra heat), but overall people are very happy across this forum.
that may be a bit hard to acquire afaik no one is selling them right now