Suspend not working appears to be a firmware issue with Debian’s firmware-amd-graphics package on version 20230210-5, replacing this with upstream’s firmware resolves the issue entirely.
Mitigation
Download the amdgpu folder from firmware-main here, then remove all files from /usr/lib/firmware/amdgpu/ and replace it with the updated firmware files from inside the archive’s firmware-main-amdgpu/amdgpu/ folder.
I got the idea to investigate this from @Mario_Limonciello 's bug here, then running the script he suggested here) helped me reproduce the issue with logs and test the replacement firmware. Thanks for the guidance @Mario_Limonciello , I really appreciate it!