The output from the buck converter should be the same/very similar when using 12v or 24v but I would suggest always checking.
The vcc out from the buck converter goes only to the vin on the Arduino (via a diode) so if you remove the Arduino the v out from the buck does not go any where. Please remember that the 12v/24v part of the circuit is still live through. This is the power to the valves and mosfets but these are not connected to the output from the buck.
Make sure you remove the power before putting the Arduino back. I have just killed an Arduino by not doing so.
Although there is a performance difference between 12v and 24v valves the reality is it is so small you won’t notice the difference. I decided to go exclusively with 24v for convenience. I live in a small apartment with very limited storage space and all my boxes of
junk electronics and water drop equipment became too much and I had to start sorting them out.