I was a bit lost as to why a non-supported system would be so hard/expensive. I don’t think it is more expensive under Nerves than any other Buildroot. Then you mentioned Yocto and it makes sense. Using the Yocto one gets you direct vendor support. I haven’t tried Yocto but haven’t heard the most positive things.
Sorry to hear you couldn’t use Nerves but glad you found it desirable 
Yes. If you do it yourself with 0 experience in the field buildroot would be easier than yocto. But a yocto-toolchain you get from all serious manufacturers. And getting the system up and running is not the main point, it also needs to be maintained. Which in our case, is also the responsibility of the manufacturer.
A wallmounted touchpanel and a custom railmount-server.
If you use any board like raspi or such, buildroot/nerves should be relatively easy.
We manufacture communications equipment, satellite and troposcatter modems. The appeal of Nerves/Elixir was the fault tolerance and small footprint (based on buildroot) compared to the Debian/Yocto type embedded Linux distros. We are not webapp people, we design hardware. The Elixir ecosystem lets us spend more time doing the signal processing and hardware dev and less time working on the various UIs (web/REST/SNMP, etc).
For a specific use case, if you ever fly on Southwest airlines, there’s a 99% chance that your in-flight connectivity is provided by a Nerves-powered modem.