NAS is a very established solution that is not expensive at all to set up at home.

Try editing 4K videos over 1GbE Internet from your NAS, because the mini doesn’t support more people 10gbe.

You still have local storage...

This is the same concept as L1 -> L2 -> L3 Cache -> RAM -> Disk.

When you need something, bump it up, when you don't, move it away.

I'd prefer to store my hot raw video footage on my disk, and my preferred way to do that is by buying a $350 2TB nvme instead of paying Apple $1000 or whatever for the privilege.

I understand that it's less convenient but you actually can buy that 2TB NVME SSD as external drive too. It should operate in the same ballpark as decent to higher end internal NVME SSD options.

Not slower, with USB/SSD speeds today.

Do you need a 512GB boot drive?