Hey, good X99 boards are pretty neat, I can understand wanting to stay with the setup. That said, I personally would not recommend Windows 11 at this time, unless you particularly want a certain feature from it. Now, I tried the Windows 11 beta/preview on my X99 system, using the workaround installer for older systems, but I ended up going back to my backup of Windows 10. Now, Windows 11 may have improved a bit since then, but I personally don't see the point in using it now unless one is on Alder Lake.
Now, if you really want to try installing Windows 11 on your X99, you could use one of the workaround installers, I think Rufus has one. No need to worry about secure boot, UEFI vs. legacy, TMP, or anything. Basically, certain workarounds allow you to bypass/ignore the system requirements. You should even be able to do an upgrade if you want, without having to reinstall!
Also, while you are at it, you may want to consider upgrading your board with a cheap Socket 2011v3 Xeon, if desired. Many X99 boards will support these with BIOS updates, and this also can allow you to really upgrade the RAM in your system. Now, YMMV depending on the board, but on my Asus X99 Deluxe, I have a Xeon E5 1660v3 with 256GB of DDR4 ECC Reg memory. The ECC is probably not active, but since I have a Xeon, I was able to install 8x32GB RDIMMs for cheap from ebay. So you may want to look into what your board can take, if you want to give it a bit of an upgrade. Keep in mind, the 1600v3 series of Xeons can OC as well. The v4 Xeons mostly cannot, but may support even larger capacity RDIMMs.