I've merged the keepalive fix to master now which means it will be in the latest beta in 15-30 minutes and released in v1.63
If you want to make a PR to make minsleep configurable you could copy these ones from the google drive backend
--drive-pacer-burst int Number of API calls to allow without sleeping (default 100)
--drive-pacer-min-sleep Duration Minimum time to sleep between API calls (default 100ms)
They don't need to be configurable in most backends but webdav has a lot of different providers.