Do you have any benchmarks with a mix of long open transactions and short ones? I've struggled a lot with WAL-E in the past there, and am curious if that changes here.

no. but wal-g & wal-rus both have parallelism over wal-e. however are you more asking about handling build up of wal / vacuum prevention caused by long running transactions? those are up to postgres, archive command only keeps pushing wal so that when postgres is ready to get rid of wal it can. seems like your scenario wouldn't care much what the archiver is since wal should be shipped long before postgres is ready to get rid of wal