I know that the number of mA is important to calculate how much silver is released.
It is indicated that at least 10V is required for this process, but between 10V and 30V (recommended power supply) there is quite a difference.
What is the ideal voltage to work with? and does a higher voltage have any advantages?
Technically, the higher voltage, the better. However, the actual figure does not matter. To understand this, you need to know how a Constant Current circuit works. As the conductivity increases (the more colloid is in solution) then gradually the voltage will decrease and the current (amperage) increases. In other words, the voltage required to "push" the current around decreases. This is what you are seeing. You cannot hold both current and voltage steady; voltage reduces as the machine puts more ions into solution.
Higher voltage is however better, as the ions are dispersed faster from the anode; the entire process is a balancing act, everything affects everything else. If it were simple, this forum (and this thread in particular) would not need to exist.