The only real issue are updates. After just 3 years my previous phone didn’t get any security updates and I had to get new hardware. I actually liked my previous phone more than my current one. But it is how it is.
You want to get into microcontroller programming somehow. The Raspberry Pi Pico may be a good target to start with. The documentation is well made and you have the choice to either use C++ or Python for programming.
The relay will cause a short voltage drop when switching. This could be a problem if your circuit can’t handle a short voltage drop.
Probably the Mean Well has adjustable output voltage. If you can trim the output voltage of the power supply to a higher voltage than your battery, then you can probably just run each power source through a diode and merge them after the diodes.
The number one question I would ask about HTTP would be: Why was the “Referer” header initially added and why wasn’t it removed from standard to this day. In my opinion the server, I’m going to, should never know where I came from.