How can I reduce the responsivness of my homebrew brake system?
the pressure to disk brake is too high?


That is sure, that is the only parameter that can influence how much braking authority you are using.

Given the schematic you posted, my otherwise uninformed opinion (as I don't have pressure measurements, or other data), is that you should investigate your brake valve.

It is probably not providing a sufficient pressure drop even when slightly open. This could be due to the valve model.

Yes, your suggestion to have a restrictor valve (or an O ring inside the pipes) should be a suitable approach, but it depends on the solution you are looking for.
The pressure will drop when the brake fluid is moving, but the equilibrium state will not change, if you keep your pedal slightly depressed your aircraft will still come to a complete stop, simply it will do so slightly later.

If your intention is to have the braking authority controlled by the pedal, what you really want is to change the pressure reaching the brake disk with the pedal.
No static solution is viable in that case: you will need a valve that lets you control the pressure received by the brake plate with the pedal deflection, i.e. you probably have to change your brake valve, or you have to install a system that dynamically adjusts the pressure downstream of the brake valve.