To be clear, I needed to build my own driver only to get Vulkan to work, as Debian doesn’t have a mesa-vulkan-drivers package for RISC-V.
Apart from that the RX550 (specifically, an Asrock Radeon RX550 2G) works out of the box with SiFive’s 5.13.9 kernel and Debian’s mesa linux-firmware packages. I didn’t have to do anything special like override kernel command-line parameters. It does hang for 20 seconds before an image comes up, I reported this here: "rcu_sched self-detected stall on CPU" during boot.