Hi Michael
Do you use Windows for video editing as well as FCPX? Or is it just sometimes you need to copy some files between Mac and PC?
exFAT is okay and the most compatible when going between Mac and PCs. I think using native MacOS Extended HFS+ formatting for the spinning hard drives is a better and more reliable format. Personally, I'd stay away from NTFS (new default Windows format) unless only for PC's not Macs. There are translation programs to allow Mac to write to NTFS, but what a pain and will slow your thru-put and cause many issues. Macs can read NTFS, but the write scene is truly messed up.
Not sure why you do not see APFS option for your ssd's?? What version of macOS are you running? APFS is available at least from High Sierra forwards. If you are only using the ssd for editing on Macs, I would see about reformatting next time to APFS. Use separate drives for PC data transfers.
WARNING Do Not Reformat without having back-ups as Reformatting will ERASE your drive.
I would not ever use APFS for spinning hard drives, not good, and Time Machine cannot use APFS and needs HFS+.
Having said that, APFS can be faster and more reliable for internal and external ssd's.
Lastly, I feel that stuffing drives into the 90% full can reduce reliability as this will increase fragmentation of files and due to the way that the OS handles adding and copying files, you risk corrupted data as the system tries to find room and the drive heads will be going crazy trying to find and write space. Having said this, I don't always follow my own advice. Do as I say, not as I do

Drives, especially Hdd's are cheap enough to not take chances.
Hope this Helps, Greg