"The problem is finding a switch small enough to coexist with the pick up tube down the hole"
No big deal. Take a look at toilet float valves in any hardware
store. The old kind used a big hollow ball on a lever arm. The new kind, which is what you want to look at, use a "donut" that floats up around the fill pipe.
You want to start with that, or copy it. Buy some high strength neodymium disc magnets, epoxy
them to the float collar. Now place a glass encapsulated reed switch in the pipe, so that when the float donut comes up, the magnets trip the reed switch, and you use that to turn on the pump.
For redundancy you can fit several reed switches in parallel so that as the donut floats up, it trips several of them all the time, not just one.
Magnetic reed switches are used in common alarm
contacts on windows or sold in electronic supplies. They're fairly waterproof (like light bulbs, the glass is sealed) but you would want to go over the wires that exit them with epoxy
or other sealant
to make that 100% waterproof.
A couple of bucks and a couple of easy hours of your time, and you can fit that switch into anything, with multiple redundancy.