We also do not recommend the Pi with an external SSD due to power delivery issues. If you are going to use a Pi, we suggest using an encasement like the NASPI. But your best bet is really an old laptop or mini PC. You can get something much more powerful than the Pi with 1-2TB SSD for like $400. In fact, in a couple days, we will be selling such a device :)
Discussion
If I remember right, umbrel had the same issue, but as long as you used the recommended hardware and the official power cable, it works fine. I followed the Start9 diy guide, and it works great. Using that now in place of the umbrel. Will maybe upgrade to the more powerful hardware in the future.
I use old office mini pc. 💪