Renaming is handled fine by mergerFS. If you read up on the docs you can see that you can assign different volumes one or more of groups of actions.
So for example you can have a certain disk do move, renames and delete, but not write any new files (which you may want to send to a local disk temporarily for later upload). Animosity knows more about the spesifics of that setup than me, but it's all in the docs basically - and you can also go to his thread and steal his won settings as a base to work from since they sounds like they are very close to what you want...
As for scans - they use API calls to list, sure, but scans are in general not a big problem as long as they scan for the basic attributes. it's when programs scan for extended metadata or actual file contents (like say - generating previews) that things get bad since that will require opening each file in turn rather than just getting a bulk listing.
I would assume that sonarr and radarr operate on basic attributes and probably won't make too many problems as long as scan settings are reasonable - but I do not use these personally so that's just my best guess. You can keep an eye on this site to get a feel for how much of your quota you are actually using and if you think you see abnormally high usage compared to expectations:
https://console.developers.google.com/