Learning how to maintain and adjust hydro brakes is a useful skill.
However, they will never be as easy to deal with as rim brakes.

If you’ve already decided to replace the brakes due to the cost of a bleed kit, I see no further troubleshooting required.

If you change your mind, let us know.

BTW, in a Shimano caliper, it’s not the fluid that retracts the pad after braking, it’s the piston seal.
A bubble won’t stop it retracting, but it will soften the braking.
not sure about Promax.

oh, and a Promax bleed kit is $26 including the fluid!

Barry