A fully charged RC/Phone will last longer than three MP batteries - so if they are both charged when you charge the MP batteries, it's just not a problem worth worrying about.
It's also easy yo use your car or a decent battery pack between flights to keep both phone/RC topped up.
I have this power bank - https://www.amazon.co.uk/gp/aw/d/B076KS92ZT/ref=ya_aw_oh_bia_dp?ie=UTF8&psc=1
Is this ok to use on the RC?
I've read some people say it's not good to charge the RC with anything other than the DJI charger.