Oh boy, this has cost me a lot of money, and i'm afraid i can predict the outcome.
The seller told me he was an expert. However i identified some flaws myseld
I should of have shared it with you guys before accepting the PCB design.
Attached is the schematic as JPG.
I would have designed this using a dedicated H-bridge driver IC. That may also allow you to use n-MOSFETs for the high side. 8x nMOS @ 1.8W = 14.4W total heat from that board. In your case: 4x nMOS @ 1.8W + 4x pMOS @ 13.5W = 61.2W to dissipate.
Do you have a recommendation for a off the shelf H-Bridge module that i could use? At least for prototyping.
The reason that i used a mega was because this design was initially intended to use the scissor jacks.
Using the scissor jacks i would have 2 switches per motor. Also i wanted to control the motors individually through the arduino (later i found that it was quite useless to do it like this) to calibrate the gyroscope. Of course i can calibrate it with manual switches directly to the motors.
Then a All-up button and a All-down button.
So because of these inputs i found i needed a mega for it.
Later on i found the hydraulic ones, that are far more powerful.
However they don't have any limit switches.
And without the toggle switches i would have far less inputs so i probably can use a nano, am i right?
Crap. So what to do now. 
