Problems like this are very frustrating and time consuming, so you have my sympathy. I also have have BIOS 1601 with all settings on defaults (apart from a slight reduction in PCH voltages for temperature improvements) and performance has been fine. I have a 12900K but just 2 sticks of Corsair 5200 DDR5 to give 32GB RAM. I also have a 3090 (TI).
It's almost impossible to say what your problem might be, but my thinking would be as follows:
a) Has anything changed - new software, video drivers, Windows updates etc. Have you installed anything that might be considered untrustworthy?
b) Have any hardware settings been changed? (particularly relating to the BIOS or GPU)
c) I have read that 4 sticks of DDR5 RAM can be less stable than 2, but your system was OK before, so probably not relevant here
d) I would run memory tests anyway as they are quick and easy (you have done that)
e) Check your cables and replug all USB & other connections. I had a faulty cable recently that was causing all sorts of strange issues. Took ages to track it down.
It's less likely that you have experienced some sort of hardware fault, so working on the assumption that everything was fine before you updated the BIOS, the first thing I would do is go back to the the previous version and take it from there. Keep detailed records of everything you try and change just one thing at a time. You'll find that such notes can be invaluable in thinking about what effect was obtained and what you might try next. A slow, methodical approach is really the only way to move forward. I'm assuming that you have a decent virus checker? If not, you also never know if some nasty has appeared on your system.
After considering anything else else that might have changed recently and dealing with that, I would be thinking about a fresh install of Windows with all BIOS settings on defaults. Install as little software as possible initially - for example, don't install Armoury Crate, just add all of your mobo drivers manually. Start planning for a fresh install while you try some simpler things first, but you can start planning and making sure that all files, passwords and data are backed up in advance in case that route is chosen. Take it slow and have a really good think about a good plan of attack. Good luck and another thing I always do is keep images of Windows for easy restores. Worth considering once you get it all sorted - a massive potential time saver for the future.
Z690 Hero, 12900K, BIOS 4001, MEI 2433.6.3.0, ME Firmware 16.1.35.2557, 7000X Case, RM1000x PSU, ASUS TUF OC 3090TI, 2 x 16GB Corsair RAM @ 5200MHz, Windows 11 Pro 23H2, Corsair H150i Elite AIO, 4x Corsair RGB fans, 3x M.2 NVME drives, 2x SATA SSDs, 2x SATA HDs.
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