Now, when cranking, battery voltage drops from 12.7 to 10.5 but on starter terminals I measured 7.5 Volts. With this, the starter runs to much faster and the engine starts as easy as when I supplied energy from the car with the car emergency wires. Never, since I have this glider, it cranked as fast and started as easily.

Maybe quit and declare success.

Or add more, nore copper.

Or measure the steady state amps to go with the similar volts and see if the loss in the wires are as expected.

The electrical parts might be working as designed and there is an extra mechanical load?