I hear your point @minorhero regarding required power/flow of canister filters. I will need to think this further, but all this input is very very helpful!...
For the auto water change system, it would probably be a lot better to put everything on a timer rather then have it fill whenever a sensor gets alerted. That way you could stop your filter at the same time and thus allow the water to flow out or into the tank without fighting the flow of the filter. I am just guessing on this, I've never tried to do what you are doing here but that would be how I would do it.
Regarding the sump/canister, I see your point regarding lily pipes (I would not use Euro bracing); but the alternative of a sump, as far as I understand, one needs to install an overflow system such as bean animal, which requires a not-so-pretty-looking box at the back of the aquarium... that one also, in my opinion, significantly impacts the aesthetics. Is there any alternative way of making the sump without the need for such an overflow box?
Regarding the water change - if sump, then it will be easy. If canister I am thinking my sketch should probably work; keeping filter "on" is a must, because when solenoid valve opens on timer, the hose is thinner than the return hose from the filter; 80% water will still continue back to tank and 20% water will go towards the drain/sewage (rough figures). The filter will push the water through this drain hose so it will help. The topping-up is connected higher up, so I'm not loosing clean water; this one comes under pressure already but then is further assisted by the flow of the canister, which pulls the fresh water along; the solenoid valve should be controlled automatically by water level because this way it will be able to continuously top up also to fight evaporation, which will probably be considerable with such a large open top tank. I will need to test the design and manually meter out the timer-vs-volume drained to set appropriate daily timer but I think this should work. Again: only in case of canister filter. If going via sump route, the solution is much easier.