If you can use open source, you can build hardware
I'm going to complicate this a bit and say "If you can use open source, you can prototype hardware"Part of building hardware is making it robust enough to exist in meat space long term. That means thinking about how the humidity sensor is affected by ambient conditions (including the packaging bag, that one has bit me in the past) and having a plan for re-calibration if drift becomes too great. That means picking connectors for your wire harnesses that can handle the number of times you expect to connect/disconnect them over the course of your things lifespan. That means tuning the length of that wire harness so you can't damage it when you open the enclosure to change the battery or whatever. It means thinking about how ambient conditions affect the rest of the design, so you don't have to clean the contacts on all the wire harnesses every so often, because you didn't get gold contacts for both the harnesses and the connectors, and you live in a high humidity environment.Don't get me wrong, I'm self-taught on virtually all of these points, it is achievable for the hobbyist. Just understand that swapping out one smart relay controller for another is pretty far from having a smart relay controller you'd even give to your sister-in-law for Christmas.