You haven’t given specifics on the hardware in question, but is the Mac mini a new machine or something old and slow? If it’s the latter, you may want to check https://forum.syncthing.net/t/optimising-syncthing-for-low-end-hardware/14885 (written by myself, in full disclosure) for some tips how to optimise Syncthing for slower hardware in general.
In particular, judging by the scan time remaining, I’d assume you’re dealing with spinning drives here, which means you may want to limit maxFolderConcurrency to something like 2 or even 1. Rotating disks don’t play nicely with a lot of threads trying to scan files in different locations, which is normally the case with modern multi-core CPUs.
Also, what I personally like to do is to split data into many smaller folders rather than using fewer large ones. This way a single large folder stuck in scanning won’t block the others from operation.