Hello,
Thanks for your quick answer!
Here are some pictures:
Here is the MAX module: https://www.adafruit.com/product/3328. Thus I do use a PT100 sensor.
The OLED display is similar to this one however it is a chinese clone using an SSH1106 chip (which is an equivalent/replacement to the Adafruit one, it does not change anything code-wise).
Yes I do have a DMM, however current reading is not perfectly significant since since I have another external thermometer hooked to it (for testing purposes, it's autonomous apart from it's PSU, not related to the Arduino nor anything else on the project). But no, it does not get hot at all, the supply voltage is perfectly constant and stable (it is an actual regulator from Mouse, not a chinese one), I also checked it with a scope.
My 12V PSU is an SMPS, and actually gives off 15.4V perfectly, so as to also power the Peltier modules as well. Here it is: https://www.digikey.com/en/products/detail/dcomponents-corporation/AMESP600-15SMNZ/17619977
Yes I do use a flyback diode, it's not on the schematic since I soldered it close to the pumps and not on my prototype PCB. I use 1N5819 schottky diodes I had laying around.
Thanks again.