Asus PRIME B650-PLUS motherboard: a disappointing experience

I am retiring from academia, so I had to burn some remaining cash and settled on building a new computer. After the latest troubles with Intel processors, I decided to look around for an AMD processor and motherboard. I wanted to have a generous amount of RAM as I have been playing a lot with different IA projects. So, I chose a 4nm Ryzen 7 9700X processor (8 cores, 40 MB cache), and I trusted the good reviews of the inexpensive Asus PRIME B650-PLUS , which I checked supported 128 HB of DDR5 RAM. The build was easy, and I was quickly running Windows 11 and Ubuntu 24.04, but something fishy was going on: My DDR5 DRAM was only running at 3600 MHz, which the manufacturer marketed as 5200 MHz. I contacted the seller's support, and I was told that while the board could do 128 GB and 5200MHz memory, it could not do both at the same time, and the BIOS was choosing a more conservative frequency for the memory if I was using the four DRAM SIMMs (which I was). I could have taken the loss ...