First of all, my go-to answer for Time Machine conundrums is Howard Oakley’s Eclectic Light Company, and he had a post on APFS and indicates that Time Machine backups are always on case sensitive APFS volumes. In a comment on a Tidbits thread on this post, Matt Sephton explained that APFS user disks are case insensitive and APFS Time Machine disks are case sensitive, so if two files have the same name, both will be backed up.
As to my backup strategy, as others have said, a lot of critical stuff is in iCloud or GitHub. But having lived through a couple of disk crashes, one before I did any backups (a long time ago - the drive was a 10MB drive in a Mac SE - and required clean room disk recovery) and the other when I only used Time Machine (and the recovery was slooow), I settled on a multi-layer strategy. I use Time Machine (SSD attached to my iMac) for incremental backups to handle a deleted file, etc. I use Carbon Copy Cloner to make a full disk image of the data portion of the drive (I used to use it for bootable backups, but that no longer makes sense). If a computer isn’t functioning when I buy a new one, this backup is used to transfer data. Finally I use a third party, off-premise backup in case of a fire or other disaster destroying all the local backups.
