A common way to handle this is to choose a version that is stable enough and has support for the feature set you’re looking to deploy and then simply not updating it. You may still have to backport bug fixes that may arise from unforeseen corner cases or uses and in case the instance is open to the internet, security fixes as well. Basically you would have an extended service release, frozen in time with a certain well defined feature set. You make your money with services around that feature set. The software is just a tool.
Then, once a year or something like that, you release a new version with all the new features.
A lot of third party software based on open source (like Linux based firmware, for example) operates like this.
Oh and for the UI, have a look at Tileboard. In my opinion, it’s one of the best looking, most functional and flexible UIs for HA. It requires JS programming knowledge though.
Oh absolutely. It’s more than just hacking together something barely functional with copy’n’pasting random stuff left and right. If you want something high quality with long term support, custom software development for commercial purposes can easily go into the 5 or 6 figures range, even on relatively ‘simple’ things (because it never is as simple as it may look on first glance).