You’re welcome. Thank you for your work on restic. 
As for the cache, good to know that prune will at least clean some things up.
(Although the prune does seem to take some time in some cases. Currently sitting at almost 48h and counting because a lot of files got removed. )
Can I safely nuke parts of the cache manually, or is there some index keeping track of things?
This machine is only supposed to run backup and forget (with --prune). Any things that I can easily automate removal off in that case that won’t have too much performance impact?
The cache has now grown slightly to 40 GB. So for 1.2 TB of data that would mean about 3%.
This is still large enough that it could be prudent to make this more noticeable to new users. ~/.cache might not have the space for it in many installations. That’s how we first noticed this issue.