FWIW - I found that editing /etc/initramfs-tools/initramfs.conf and setting COMPRESS=lzma and COMPRESSLEVEL=9 was enough to get initrd to fit in my /boot. Using lzma instead of zstd for this particular release changed the size of the ram disk from 229,092,670 bytes to 86,150,862 bytes. Of course it takes longer to create.