If you prefer the solenoid to be normally OFF, then just trade locations of the opto coupler and resistor. That way the solenoid will be OFF if disconnected from the Uno or when the Uno is powered down.
You're missing a series resistor to the IRLED, 220Ω would work well.
I changed my schematic accordingly (hopefully) so that the solenoid is turned off by default, and the arduino signal turns it on. I don't understand where I'm missing a resistor (and why).
I didn't see OP ask for an opto isolated design.
Downside from that design is that the solenoids are ON by default (when the Arduino is off or booting).
The diagram from post#0 (plus ground) is perfectly ok for ~1Amp solenoids and e.g. TIP120 darlingtons.
Supply can be increased to ~13.5volt if you want the full 12volt on the solenoids
(darlingtons have a high-ish saturation voltage).
Four logic level mosfets, 220ohm gate resistors, and 10k pin to ground bleed resistors would be the best option.
Almost the full 12volt available for the solenoids, and no hot transistors.
You're right, initially I didn't ask for a new design with an optocoupler, just a check if my topological design is correct, and advice on how to chose the right components. As this is my first project, I didn't knew of optocoupler before, so It's no shame that I got introduced to this idea and learn how I can protect my arduino with one. It's nice to be reassured from someone, that my initial work isn't flawed, and that all this extra stuff isn't needed to make it work... Thank you!
The newest schema (with a pull-down resistor?):