Nice to see someone try this.
I have done a full cruise control this way a few years ago and would like to share some tips. I don't have any code to share or pictures.
I used a software PID-control on the Arduino to regulate the throttle. The input signal was RPM, not speed (manual transmission). That way if a gear popped out, it didn't rev itself to death. So it was like a constant rpm control. On automatic transmissions you have to use the speed signal also, and implement rpm limits to stop operation. I used an aftermarket cruise control actuator, which had all the gearing for the motor and cable control. I got it from a junk yard, and was very cheap. The worm gear structure makes life easier for the H-bridge, as it only has to supply power when the motor moves. No holding current is needed. It also had a safety magnetic clutch, that disengaged the cable from the controller motor in case of emergency (had to use it a few times when testing). All in all, a very nice and interesting project, although the gas I spent adjusting it all ended up costing more than a full cruise control kit from the junk yard.
Judging from your pictures you have a Bosch Monojetronic setup, in which case you could also try using the original throttle actuator motor with some electrical trickery.
Best of luck!