It might be preferable to run the primary OS on the SATA disk instead of the microSD card. I would be interested in the following tests:
Copying the disk image of the microSD card to the SATA disk (e.g. using dd) and making it primary. Then the microSD card can be wiped clean and used for things Samba shares which are infrequently accessed. (Some people got 128 GB microSD cards during Christmas season last year, so the storage isn’t negligible as compared to the SATA disk).