In the end I bought two AM4 boards and one AM5 board.
The AM4 boards (an Asrock A320M-HDV for the A12-9800E and an Asrock Fatal1ty X470 Gaming-ITX/ac for the 2400G) are there for the CPUs that were pushed out of their mobo's in favour of better CPUs/APUs. Got them dirt-cheap from a company (Dealstunter) selling left-overs and refurbished equipment -my earlier X370 iTX is also from them. This gave rise for a need of new cases, mainly in the iTX department.
So, I also managed to get hold of a 2nd hand white version of the Fractal Design Node 304 for the Fatal1ty X370 Gaming-iTX/ac based 3400G that got as comment from the wife upon seeing it "have you bought a new microwave?". You can see in the link how easy it is to mix up the two!
I am now saving for a 2nd Node 304 in black, for the Asrock Fatal1ty X470 Gaming-ITX/ac, as they fit so snuggly in my computer rack -being just 210mm high. They also have room for a decent sized (165 mm high) cooler, plus two 92mm intake fans and a 140mm exhaust fan. The ideal iTX crunching-case after the Cooler Master HAF (High Air-Flow) Stacker 315F went out of production, IMHO.
The A320M-HDV, that I had planned to go with the with the Athlon A12-9800E (those guys at Dealstunter sell no sh#t when they say 'updated to the latest BIOS' -they did! BIOS L8.01 in fact. So: No Bristol Ridge support as consequence. And "ASRock do[es] NOT recommend updating [to] this BIOS if you are going to use [a] Pinnacle, Raven or Summit Ridge CPU on your system.....") will for the time being re-use an old Packard-Bell casing that used to be the PC-case of my late father and that once held an AMD Sempron as the casing sticker still proclaims.
As the Athlon A12-9800E now definitely gets retired -I simply have no board left supporting it!- the A320M-HDV board will get the Ryzen 3 3100 and will be the core of a future Windows 11 system for a blind friend of my wife and me, for whom I do the IT-support -she is now running Windows 10 on an Intel Core 2 Duo E8500, which is still enough to run JAWS for her.
The AM5 board then.
Just as with the introduction of the AM4 platform, when I went for an APU with the board that gave all I needed in terms of upgradability and APU support,I have now gone for an ASRock A620M Pro RS, and this with the Ryzen 7 8700G, that not only has a decent GPU but a 16 TOPS NPU as well. Now I am hunting for a Fractal Design Node 804 to give it a home. Already love to hear the wife's comments on this one..in comes my trusted, and now fully upgraded ASRock A320M Pro4 (to BIOS 7.40, so no Bristol Ridge support either).... I have to buy a Ryzen 5 5600G or Ryzen 7 5700G to still get the most out of it
My next AM5 board will be a full-size ATX ASRock B650 Pro RS, again with a Ryzen 7 8700G to see any performance differences, if any.
In the meantime I am building a new AM5 system for my son too, using a ASRock B650E PG Riptide WiFi with a Ryzen 5 7600 as CPU.
He wants to be prepared for PCIe 5.0 x16 GPUs and really full-speed NVMe PCIe 5.0 M.2 drives and does not want to change his mobo every five years -he comes from an Intel Socket 1155 system that I built for him ages ago.