Mesh flaps are useless . Rats mice, snakes are smart and smell water and climb up and around and open anything and so do mice.
Just slip on overflow on a 1 more feet of pipe capped at the end and cross drill 20 15 mm holes in it . Yes have it off the ground as bloody ants and insects are the other issue and just have some 50mm ballast for it drain on so no erosion. You can also do a water S trap in it that stops insects
As for the other household member leaving taps on or a blown pipe - put a water outlet half way up your tank depending on size and usage and connect pump to that. You can have a 2 way valve and choose to draw from halfway down or the bottom . Yep I have it padlocked so it is not a simple thing just to switch and forget to switch back.
Next one - if you have multiple tanks have it so they are independent to pump from and don't self level. Ie have inlet so it flows from one to the other but do not have the bottom taps connected and ever always turned on. You will only ever lose water from one tank that way if something fails.
Cleaning sludge - can be nasty so never do it alone and never do it by getting in a tank. another pump to suck out sludge with hose on a pool pole. Some sludges with lots of leaf matter in large areas with right bugs can give off sulfide gasses and you don't want to be in there. . A lot of sludge is dead algae as so many tanks allow light in and that creates issues on translucent polly tanks - not so much with concrete. .
I have seen this used but never game in case siphon hole didn't work but would add a large valve so only when I knew tanks were going to overflow I would turn it on. Some silt is said to be good
I've seen too many silly mistakes with drinking tank water with wastage and bad management . How more people don't get sick amazes me as all the old rules get forgotten.
