As of now, i keep them basically for ever, but granularity gets less over time. (restic forget)

Before you go nuts about files you want to exculde the “churn” as you said it, go ahead and actually check how much space this actually takes up. Text and especially logs are highly compressible and will most likely end up taking up almost no space.

Be sure you backup everything you actually need, rather backup a file more then a file too less.