Hi mate the design of an ESC with a power limiter is not an easy task.
A purchased ESC will probably have a means of limiting torque or current, but not power. Battery current is probably the best indication of total power delivered by the battery. However a given current level will result in less power as the battery voltage declines. Total battery power is power used by the propellor plus motor losses plus ESC losses.
Answer 1: The pragmatic approach:
You could test a motor-ESC-propellor combination at various speeds then limit the speed to whatever results in 700 W use, you may find various simulators online, like this one: https://rcplanes.online/calc_motor.htm .
See the simulation for a 700 W max system above.
Answer 2: The electronic engeneering approach:
since the instantaneous electrical power is given by the instant voltage*the instant current, we should measure both and limit the output using a custom designed limiter: The result of the battery power estimate or calculation could be used to reduce the speed command when the power exceeds a set limit (in our case 700 W). The system would essentially prevent the propellor speed from exceeding the speed that results in the set power limit whatever that speed happens to be under the prevailing conditions.
Moreover there is a similar discussion here: https://electronics.stackexchange.com/questions/594444/how-to-do-a-power-limiter-for-brushless-motor
Happy to go in more depth if you may need it.