you say it complicates auto-waterchanges => the matters if you turn off your return pump , because you will not know the amount of water will ato-ing.
This is incorrect because you do know the quantity ( it will always be the same (from max water above ato , to minimal sensor ) because of your ATO that was active before the auto-water change keeps its at the same level so the cycle will always start with the same amount of water so you don't need a level to the top quantity , ( as long you you fill long enough with new water, time the time it takes to let the pump reach the level + add some extra seconds.
after switching on the return pump after the ATO water change , if you over filled ( small amount , since you timed it and you will never over fill since the timer is prevent that )
its even better to under fill a small amount , your ATO will top off this minimal amount like it would normally do.
Xiaans solution is KISS and its proven its works , yours seems to be very complex and might be impossible with the profilux. ( switching functions of level switches depending on states
also big water changes , or 2 smaller ones , its doesn't make alot of difference. Both have pros and cons.
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