don't sleep on the jonsbo n4 either, Its a good price (in america, can't say about europe) but the main thing is that it can use matx sized motherboards which tend to be less expensive than itx and has slightly more connectivity.
BIgger cooler is probably not gonna be the biggest deal for you but bigger coolers can run quieter and ironically good ones are cheaper than decent low profile coolers.
Consider splurging a little for a psu that is fully modular because its a lifesaver when working with small form factor cases.
good catch picking a motherboard that doesn't use a realtek nic, even i've made that mistake.
I can't really check prices for where you are at but in america i can find a i5-11500 for cheaper than the i3-12100 and the 11500 is superior.
Also buying used for intel cpus tend to be safe.
I would skip the noctua unless it is going to be placed somewhere you can regularly hear it.
even then i would check the prices of a thermalright cooler first. They cool just as well and are almost as quiet for less money.
Looks like you've already thought of some of my suggestions but i'll post them anyways
Everything is expensive these days, but my suggestions are to get used cpu and ram
you can also cheap out on the case and get a generic case that isn't nas specific like the jonsbo.
also try using pcpartpicker i think it might work for austria