If your device is working stationary without permanent attendance and must not be optimised for lowest possible weight I would use not a lithium-polymere battery. (LiPo)
If you overcharge any Lipo the lipo wil blow up in fire.
I would use a Lithium iron phosphate battery.
A Lithium iron phosphate-battery is safe in itself and does not go on fire.
If you still insist on using a LiPo I would buy a ready to use charging module To have some safety. I guess you don't want to go through a certification process that your DIY-PCB fullfills all safety-requirements professional charging circuits have gone through that were used in smartphones.
There is a reason why cheap chinese hoverboards go up in flames.
best regards Stefan