The migration is done and the system booted again and seems to work fine. I kept the locking of glibc in place because Leap 15.3 and 15.4 use literally the same glibc version (as the package is inherited).
Only two services failed: sshguard.service and zramswap.service
martchus@ariel:~> sudo systemctl status sshguard.service
× sshguard.service - SSHGUARD provides automatic attack blocking
Loaded: loaded (/usr/lib/systemd/system/sshguard.service; enabled; vendor preset: disabled)
Active: failed (Result: exit-code) since Wed 2022-07-20 12:57:07 UTC; 4s ago
Process: 12381 ExecStartPre=/usr/sbin/iptables -N sshguard (code=exited, status=0/SUCCESS)
Process: 12382 ExecStartPre=/usr/sbin/ip6tables -N sshguard (code=exited, status=0/SUCCESS)
Process: 12383 ExecStart=/usr/sbin/sshguard -a $THRESHOLD -p $BLOCK_TIME -s $DETECTION_TIME -w $WHITELIST_FILE -b $BLACKLIST_FILE (code=exited, status=78)
Process: 12384 ExecStopPost=/usr/sbin/iptables -F sshguard (code=exited, status=0/SUCCESS)
Process: 12385 ExecStopPost=/usr/sbin/ip6tables -F sshguard (code=exited, status=0/SUCCESS)
Process: 12386 ExecStopPost=/usr/sbin/iptables -X sshguard (code=exited, status=0/SUCCESS)
Process: 12387 ExecStopPost=/usr/sbin/ip6tables -X sshguard (code=exited, status=0/SUCCESS)
Main PID: 12383 (code=exited, status=78)
martchus@ariel:~> sudo systemctl status zramswap.service
× zramswap.service - Service enabling compressing RAM with zRam
Loaded: loaded (/usr/lib/systemd/system/zramswap.service; enabled; vendor preset: disabled)
Active: failed (Result: exit-code) since Wed 2022-07-20 12:56:50 UTC; 4min 11s ago
Process: 12011 ExecStart=/usr/sbin/zramswapon (code=exited, status=255/EXCEPTION)
Main PID: 12011 (code=exited, status=255/EXCEPTION)
Jul 20 12:56:50 ariel zramswapon[12038]: swapon: cannot open /dev/zram3: No such file or directory
Jul 20 12:56:50 ariel zramswapon[12039]: swapon: cannot open /dev/zram4: No such file or directory
Jul 20 12:56:50 ariel zramswapon[12040]: swapon: cannot open /dev/zram5: No such file or directory
Jul 20 12:56:50 ariel zramswapon[12041]: swapon: cannot open /dev/zram6: No such file or directory
Jul 20 12:56:50 ariel zramswapon[12042]: swapon: cannot open /dev/zram7: No such file or directory
Jul 20 12:56:50 ariel zramswapon[12043]: swapon: cannot open /dev/zram8: No such file or directory
Jul 20 12:56:50 ariel zramswapon[12044]: swapon: cannot open /dev/zram9: No such file or directory
Jul 20 12:56:50 ariel systemd[1]: zramswap.service: Main process exited, code=exited, status=255/EXCEPTION
Jul 20 12:56:50 ariel systemd[1]: zramswap.service: Failed with result 'exit-code'.
Jul 20 12:56:50 ariel systemd[1]: Failed to start Service enabling compressing RAM with zRam.
I don't really now those but both are provided by official SLE15SP4 packages so they should have been covered by the migration to Leap 15.4.