There are two problems you are likely to deal with. First, small computers tend to have lower-end CPUs relative to a larger laptop. But, any modern x86 processor (e.g. the Meerkat) is going to be faster than a Pi.

Second, parts for smaller computers are often harder-to-find. For instance, power supplies for small computers tend to be specific to that model. Whatever you look at, check reviews to see what fails and look into what it costs to get those spare parts. And maybe buy those spare parts before you need them.

I suspect that will be more reliable than a Pi, but I can't say for sure since I have no experience with System76 machines.

Definitely get the tall version, since that will let you install an ordinary 2.5" SSD.

I recommend going with Ubuntu over Pop!_OS.

The installation process for Umbrel seems straight forward to me. You just run an install script via the command line. But, I'm probably biased; I've dealt with some very difficult/complicated installation processes (e.g. fighting ancient source code).