Advice for migrating a small Mac network from Mac Server to NAS

I don't know of any NAS, with any RAID configuration, that approaches server-level file speeds, especially when usage goes outside basic "full open/full save" modes. The sort of interim modes where (for example) Adobe does selected reads, controlled by the app host, to do previews and searching and such, seem to be beyond the small, fairly dumb controllers of NAS.

My Win server is around fifteen years old, running Win Server R12 with a high-end RAID controller of (IIRC) mid-grade discs. (That is, not 10k high-transfer-speed server drives.) I have never had a file access or management issue with any app, nor any noticeable access delays. I keep ALL work files on it other than local junk, and never move files to local drives for work or access purposes.

It doesn't take much server power for a few users, but it has to be a full-fledged server, with a full server OS that can manage more complex file access and operations than "in and out." That model works fine for office files of moderate size, but it's always been a bottleneck for apps that use many or very large files — most of the Adobe suite, for example — or need fast segmented access, like most database managers. The controller and OS just don't have enough horsepower to manage that complex file access with any speed.

I have an old Buffalo NAS that I've used almost exclusively for archival storage and backup. It was always adequate for write speed, but deadly slow in returning anything but smaller files. Now, it uses obsolete file protocols but is still good enough as a backup destination.

I'd say that any shop with multiple workstations that needs to share files, as well as keep them centrally managed for versioning, sharing and backup, is being penny-foolish not to use a full-fledged server for the job. Besides the slow and erratic access by advanced apps, the whole process of moving files and packages back and from (say) a NAS to local is fraught with time, data loss and human error land mines. And I may be out of step, but I put most cloud storage in the same basket. Whatever is "gained" by shareable remote storage is often lost to limited file operations and the dangers of that "check in/check out" work model.

ETA: Put the cost of  a "good" NAS into a new server — Mac, Win or even Linux. Maybe a good used one that someone outgrew, or that needs a new disc array. Any platform can be optimized for use in a Mac environment, but all will take some expertise to set up and a bit of routine supervision and maintenance as time goes by. But it's the right solution, no matter how attractive a "plug and play" NAS might seem. Use any existing NAS or the old server for storage, such as backups and archived client files.