1. It depends of what you want as future proof (expansion capabilities). Usually home user nases come with low power cpu, a high power cpu usually is a enterprise grade nas, really costly for a home user. So having it separated makes the cpu upgrade easy but now you have 2 boxes. But if your terramaster comes with a decent cpu I don’t see any problem.

2. True nas scale is really a behemoth able of almost everything. I would start with something more reduced like omv or unraid. You really don’t need the advance enterprise features of that and it will add only complexity to the setup.

3. If you use NFS for exporting folders from your nas, the “computing box” will see this as a local folder, so no need to have 2 copies of the same file.

Hope it helps