The max-unused percentage feature is well worth it to 80/20 the prune process and only prune the data which is easiest to prune away (i.e. not try to remove small files big packs but focus on packs which have lots of garbage).
In general, there's an unavoidable trade-off between creating many small packs (harder on metadata throughout the system, inside restic and on the backing store but more efficient to prune) versus creating big packs which are more easy on the metadata but might create big repack cost.
I guess a bit more intelligent repacking could avoid some of that cost by packing stuff together that might be more likely to get pruned together.
In general, there's an unavoidable trade-off between creating many small packs (harder on metadata throughout the system, inside restic and on the backing store but more efficient to prune) versus creating big packs which are more easy on the metadata but might create big repack cost.
I guess a bit more intelligent repacking could avoid some of that cost by packing stuff together that might be more likely to get pruned together.