I'm thinking that just the switch to 3s is going to do exactly what you're after. This esc
https://www.amainhobbies.com/hobbyw...MIqbacmryC4QIVHCCtBh2eog8cEAQYASABEgI6F_D_BwE will give you the ability to run 2s-4s, and has a 6v/4a bec (a little higher voltage to the servos than the stock esc, and double the amperage, so you *should* be able to run both of those 20kg servos without brown outs) and it's got all the programmable features of the ever popular wp1080 esc, just with dual motor outputs.
Your nimh is the same physical size as a 5000mah 2s/3s, so you could either go with a big lipo, or drop down to a smaller/lighter lipo and STILL have longer run times. For example, 2000mah 7.2 nimh has 14.4 watt/hours, and a 2200mah 3s has 24.4 watt/hours, so nearly double the runtime, nearly double the available power, at about 1/2 the size/weight. Remember, higher voltage equals higher peak rpm of an electric motor, so going from 7.2v to 11.1v will give a nice increase in top end too.