Overall, it looks like you’re done your homework, covering the major concerns. What I would add is that keeping an RPi cool is a consideration, since without even a tiny heatsink, the main chip gets awfully hot. Active cooling with a fan should be considered to prevent thermal throttling.

The same can apply to a laptop, since the intended use-case is with the monitor open and with the machine perched upon a flat and level surface. But they already have automatic thermal control, so the need for supplemental cooling is not very big.

Also, it looks like you’ve already considered an OS. But for other people’s reference, an old x86 laptop (hopefully newer than i686) has a huge realm of potential OS’s, including all the major *BSD distros. Whereas I think only Ubuntu, Debian, and Raspbian are the major OS’s targeting the RPi.

One last thing in favor of choosing laptop: using what you have on hand is good economics and reduces premature ewaste, as well as formenting the can-do attitude that’s common to self hosting (see: !selfhosted@lemmy.world).

TL;DR: not insane