Waterski, the auto-fill/drain system is SUPER ripe for disaster.
That's why I built in so much redundancy/checks/balances.
In an ideal world you'd have a simple mechanical system, yes. i.e. the system physically cannot hold more than X water level because you have an overflow on the sump that dumps straight into a floor drain. The input side is IS mechanically restricted - but actually seeing it work in real life will be a different thing. How will the RO system deal with the back pressure?
The reason why I didn't take this route is that very few people will have the ability to dump into a floor drain (myself included).
A lot of how I designed my setup is based on what I figured to be 'best practices' for aquarium design. e.g. it's hard to argue there is a reason to NOT do the beananimal drain system from the perspective of 'which system is the most reliable/effective'.
Similarly, it's hard to argue against a sump from the same perspective. ALL aquariums benefit from a sump, period.
Now, you can argue cost/difficulty/complexity/etc...but this is a DIY thread.
If you wanted a simple display tank w. canister filter, you wouldn't be here.
That all being said, I will certainly draw up the mechanical drains design and have it as an alternative - it is a great suggestion! My only caveat to the mechanical system is that you would still want flow meters on the input and output pipes. The entire system will work at its best with the most input data to work with.
Further, noted on the fert system - that's 'already a thing' that I will pirate from others. Many are hesitant/simply don't post the source code though. It will add a whole other level of complexity that may or may not be worth it. For example, that $80 unit off ebay does a lot! Except notify you of low levels, tell you how much it's injected over X period, tell you if it's stuck on or off, etc etc....
I'd like this project to simplify the understanding of what systems there are in an aquarium, plus make it easy for folk to pick and choose what automation components they want, plus provide a library/compendium of X device plus Y sensors = do this.