I noticed that vgamem_mb was still low (32 MB). So I changed to this (slightly wasteful) command-line and am now running the latest kernel (5.15.0-3-amd64):
-vga none -device qxl-vga,ram_size_mb=256,vgamem_mb=256,vram_size_mb=256,vram64_size_mb=256,max_outputs=1 Will report back if that helps.
