If you create a volume with those settings in Disk Utility, selecting that volume in Time Machine settings removes those values. You can not set reserves and quotas for Time Machine APFS volumes. While partioning HFS+ drives in the past was straightforward, APFS reserves and quotas also revealed some surprises. Any attempt to select an HFS+ volume creates a new APFS container with a new APFS volume instead. If you want them at all, encrypted or not, APFS is the only way. The same (and more) applies to Time Machine backups. If you want to change the password, you need to decrypt it and then use APFS with encryption afterwards. You can still decrypt HFS+ volumes, but that's it.
Disk Utility does not even list Mac OS Extended with encryption as an option any longer and the other way to do this so far – right-clicking the volume and clicking encrypt – also just creates a new APFS container with encrypted volume, partitioning your drive between HFS+ and APFS. Any attempt to encrypt an HFS+ volume, will result in a new APFS container (with encrypted volume) to be created.
I've been using external drives formatted as Mac OS Extended Journaled (HFS+) with encryption for a while and noticed that Big Sur can not create volumes formatted as encrypted HFS+. I thought I'd share some observations I made after having recently updated to Big Sur, with some big surprises regarding external drives.