Dimmers don't tend to like DC.
Diodes work, but the volts sag with discharge.
How about an adjustable power supply? Something that lets you do 6-12v should do.
If you've got an amp clamp, try to *really* load up the machine hard and see what it can pull from the battery, then add on to that number. If you can make it pull 10 amp, go with something capible of atleast 15a.
One of my models has (4) servos that can stall at 2.5a each, so you need to bring enough current for everyone. it's a shame you don't have specs on any of the components or we could give a real answer.
Something like this, probably the 12a or another bigger one
https://www.amazon.com/DROK-Numerica...RG9?th=1&psc=1
If you want to cheap out and go plugged into the wall outlet, you can hack an old computer power supply with a little creativity.