Backup Failed - Total Size Limit Exceeded I presume this is the first full backup you're performing. Let me know if that's not the case.
There are several points to keep in mind here:
1. The software can't determine the final size of the backups before compression, so you would only be able to keep 1 version of data on storage max, even in compressed state
2. The same can be said when backing up disks that are large in size but are half full, the final size will be smaller than the whole disk, but all of those operations are performed during the backup operation.
3. Currently purging is only possible upon successful full backup, so when you already have something in your bucket it won't be purged before new versions are uploaded.
So the first thing that I can suggest is to split that D drive into 2 logical ones.