Avoiding circuit overload with primary + battery backup sump?
By sizing the loads appropriately
Every load has a nameplate or published draw.  You are to use the numbers on the various nameplates to assure that you are not overloading the circuit, or (more literally to Code), to assure you provide large enough circuit(s) for your loads.
You can get any ampacity of battery charger that you want.
You seem to be looking for an electronic solution; that's grossly over-complicated compared to simply sizing circuits (or appliances) correctly in the first place.  But it is permitted in Code, mainly used for &quot;load shedding&quot; to allow an automatic transfer switch with a generator that is too small to pick up all provisioned loads.  You couldn't use that gear here because it is monitoring for sagging voltage or AC frequency from the generator bogging, and that won't occur on a utility-supplied circuit wired with proper wiring.  Unless the wiring is extremely long.
The DC pump is not powered by AC, it is powered by battery.  The AC battery charger which refills the battery, does not need to be large enough to carry the full power of the pump, and it does not have a startup surge. So it might be 2-3 amps, leaving 12-13 amps for the AC pump.