As others have said, running out of motherboard SATA slots doesn’t mean you need a new machine to support expansion.

You can get m2 adapter slots for more SATA drives.

If you think you’ll be building a NAS in the future, and are cheap like I am, you might consider getting a pci-e expansion card for SAS rather than SATA drives. They’re backwards compatibile with SATA drives, but open you up to being able to use SAS drives which are common in enterprise data centers. You can get used lots of those drives on eBay WAY cheaper per TB when the data centers hour them out.

I’ve got a machine with 16 SAS drives running the unRaid OS, and I’m very happy with it for data hoarding and media serving. The drives (with shipping) cost $5/TB.