I don't think you can get away with having partial support for a modern CPU in BIOS. Furthermore, I think the solution you've proposed is entirely too complicated and the problem has already been solved in a sense. The issue stems from the fact that the AGESA code for Ryzen 9 3000 series processors is massive and some of the BIOS ROMs out there are too small to accommodate that and the GUI. Keep in mind it isn't just the GUI that these boards are losing. Some are having to drop RAID support as well.
Keep in mind, the technology to resolve this exists and has existed for several years now. The motherboards that support blind flashing type features have an IC integrated into them that allows updating the BIOS without a CPU or RAM even being installed. Alternatively, motherboards with dual BIOS ROMs could literally have a different AGESA code on each BIOS ROM. Removable BIOS ROMs and a couple of different included options could work too. Lastly, motherboard manufacturers could stop being cheap and buy 256Mbit BIOS ROMs instead. Obviously, some motherboards use the IC needed to flash without a CPU or RAM installed and some have 256Mbit BIOS chips. That's the best solution, but it costs money.
This is something you see primarily on mid-range and lower end motherboards. The more expensive boards often have 256Mbit ROMs and the ability to flash without a CPU. Those are typically the ones you will see with dual BIOS ROMs as well. These are not the boards effected by the AGESA code updates.
I suppose that makes sense. Don't buy the cheapest motherboard you can scrape by on. There are motherboards I don't buy any longer based on experience. I think the people buying the cheapest motherboard available for their CPU are learning lessons. And this is NOT a bad thing.
Why would I say that?
Because that's part of how I got started in IT. This was.. WAY back by many's standards. I had a highly limited budget and got the best I could get for the money I could scrape together. Over the years I've learned a LOT by making that hardware work. And in truth it's led me down the path to a career in IT.
So to those late teens that are struggling with these BIOS related problems. Hey at least you're not having to do the interrupt dance, and weighing the cost / interrupt useage of a sound card to the cpu impact of offloading sound processing onto a dedicated device. Of course that was during the era were 30FPS was ok, and 60 FPS in modern title was considered the ****! This was when a 100mb demo made the news on the tech sites because it was HUGE. This was back in the day when a computer might come with a 80mb harddrive. (Yes I meant megabyte). And a 1 gig download would take AT LEAST an hour or two if you were lucky.
Struggling to make your minimum spec computer run as fast as possible so when you brought your PC to the LAN party you could play with the others.
Having to put an order of loading drivers in your config.sys file so your mouse driver would load into the high memory area and your optimized the block space in your upper memory to fit as much as you could so dogs4gw.exe would be able to allocate as much memory as possible to play the new tech FPS game.
When 3d was new.
Now the kids getting into computer gaming... are just computer gamers. They don't need to follow the CPU story, or eek out every last bit of memory from their systems. Windows for the most part does a passable job at all of that. The ones running custom linux builds are the next generation coming up in the world of IT. Part of why windows is embracing that kernel so strongly as of late too. The home grown tech nerd is more likely to know how to make your fire stick get all of the streaming for free than how to tweak your computer performance based on what you have.
Now nerds are more concerned with a 240hz monitor that young nerds can't afford. (I'm in that concerned group just not feeling it for the monitors yet.)
The next generation of IT folks coming up don't know the hardware. Not unless they seek a specialized education. The hobby tries and it's going to be the generation of IT without funds that are working weekends and summers to build up enough money for the new cpu and motherboard that are going to be in my shoes in 20 years.
I uhhh.. i went off on a tangent and I don't even know where I was going.
Ok I'm done... errr.. **** age.