Rate my first 2-layer board
Nice job diving in to board design

Specificaly if I should go with two grounds (one for 12 V, one for 5
V) or just one big ground plane.

Typically you think about separating ground planes for high frequency digital vs lower frequency analog, not necessarily based on which supply. I'd suggest one big ground plane. You can do the mental exercise of &quot;where is current flowing in the ground plane?&quot; and see that the 5V and 12 V return take different paths, and may overlap a little bit - but cross talk between supply voltages isn't something to worry about in general (you more worry about &quot;is this a high frequency return path that will have capacitive coupling/cross-talk with other signals or add noise on to the ground plane?&quot;

Another thing is the layout of the capacitors for the 7805 [U1] (12 V
to 5 V).

Looks good. Something you can think about is putting a start capacitor at each motor. This way at start-up they can draw the current from that cap rather than through traces since motors draw a lot of current at start up. Probably doesn't matter much in your design, but it's good practice. Some motor datasheets may tell you a suggested start capacitor, or you can estimate it based on peak/starting current draw and how quickly it reaches full speed. Current times time is total charge, and charge divided by voltage is capacitance, so start current times time to reach full speed divided by supplied voltage is a rough estimate for the order of magnitude of the capacitance needed.

Should I add some more vias to the return path after the FET [Q1-3]?
And/or generally add more vias? The idea of adding more vias is to
have enough more contact between ground and Vcc.

There are online calculators for finding how much current can flow through a via (stuff like: http://circuitcalculator.com/wordpress/2006/03/12/pcb-via-calculator/). You can check to make sure you have more than enough current when starting (peak current). If you have space, it doesn't cost you (much, if anything at all) extra to put more vias in.