For the firmware, can you please show me the output confirming which firmware you already have?
For using the kernel on Fedora: this is not what I would regard as a supported installation, it is a hack. What you can try to do:
a) unpack the 5.9 Debian kernel package in a temporary location (use dpkg -X)
b) repack it into a tar file
c) copy the tar file into a Fedora system
d) untar the file in Fedora
e) manually copy the files into the correct places on Fedora, for example, copy the vmlinux and initrd file to /boot and copy the modules to /lib/modules, be careful not to overwrite any other files in those places or Fedora may not boot again
f) on the next boot, in petitboot, manually edit the petitboot settings to use the vmlinux and initrd file
This is unsupported from either Debian or Fedora but it may let you test. In the Fedora environment, everything else will be much newer than in Debian, you will already have newer mesa, newer Xorg, newer firmware, maybe the RX 5700 will work.
Fedora's wiki provides
instructions for building the kernel as an RPM, this might be a better way to proceed. It is not very difficult. The only thing you need to change in the official package is this one line:
CONFIG_PPC_64K_PAGES=n
According to search results
people did get the RX 5700 working on Debian buster on x86 / 4k. One of them
mentions firmware. You can try to download and install the firmware update like this:
sudo apt update
sudo apt install -t buster-backports firmware-linux-nonfree