Yes, works with Monterey. Any of the AMD Polaris cards work natively but not the XFX brand of RX560. Has an incompatible Vbios that would have to be flashed. Most other new RX 560s and 570s cost well over $300 these days.
Both BIOS F5 and F6 are okay, although F6 may be trickier to find. I've been using BIOS F20 for the past few days with no ill effects other than USB-C devices being unable to connect on Thunderbolt ports. Note that Thunderbolt devices still connect and work properly. Thunderbolt devices also hot plug properly.
Yes this will still work with BIOS F5, F6, and F20. It may work with F21b, but I have no direct experience with that version. AppleVTD will need to be enabled, which is the default when using the latest OpenCore EFI from this thread. We only need to enable VT-d in BIOS.
Regarding GPU, the Intel iGPU can certainly be used and is also the cheapest because it's already there in the CPU as long as you avoid the "F" series processors (i.e. any Intel processor that ends in letter F). As for discrete AMD GPUs, it's just a horrible pricing mess right now so I won't offer any advice that lines the pockets of price gougers.
There are no practical differences between the two, but because you have a Comet Lake processor it would be "nicer" to use iMac20,2. It's okay to just change the system product name without changing serial numbers, but if you've already using iMac19,1 with those serial numbers, it's best to remove that computer from your iCloud account ahead of time.
hah!
The BCM94360NG is a Key A/E M.2 card. It should be possible to install that directly into this:
The link leads to another kind of product. Can you check it again?
