Alright, I've given it a bit more thought and I'm pretty sure now that I'm going to buy the 5800X.
I've looked at the B450 options you listed, couldn't really find the Mortar consistently but the Tomahawk seems good. My only question is about the two versions I found in ATX, being Max (7C02-020R) and Max II (7C02-014R), is there a difference that justifies the slight price gap (~5-7 euros) ? I couldn't really notice one myself but I'm no expert.
For the cooler, since PBO might be interesting as you suggested, and since ideally the kind of temps I would want to keep are like a max of ~70-75°C at full load for a little bit of durability (if that's a stupid goal feel free to disagree), I've mostly been looking at the be quiet! Dark Rock 4 and Noctua DH-U14S, which I can find for 59 and 68 euros respectively. That is, two of the best single tower coolers, and midrange coolers on the global scale as you suggested. Do you have a recommendation between the two ? Or do I need a dual tower cooler like the Dark Rock Pro 4 or NH-D15 instead for that temperature goal ?
Also you mentioned "4 SR DIMMs" but I'm not entirely sure what that refers to precisely ? Is it not the classic 4-slot DDR4 DIMM that seems to be pretty standard and is to be found on the Tomahawk like on most motherboards ? Sorry about being completely ignorant on that part.