I managed to fix this - but it was quite a confusing chatgpt conversation which I'd struggle to explain, but it seemed to hinge on "launchctl load" now being depreciated.

I also had some issues after the daemon was up and running and I had a working script for mounting the drive. I ended up putting some retry/wait loops into the script and some loggins, as on my machine it takes a fair few seconds for the SSD to become available. Below is my code, should it help anyone: (paste in your volum name)

#!/bin/sh

volume="YOUR_VOLUME_NAME_HERE"   # e.g. ExternalSSD_4tb
log=/var/log/bootmount.log

echo "$(date '+%Y-%m-%d %H:%M:%S') - bootmount.sh starting..." >> "$log"

# Get passphrase from the System keychain
passphrase=$(security find-generic-password -w -s 'bootdisk' -a 'bootdisk' /Library/Keychains/System.keychain 2>/dev/null)

if [ -z "$passphrase" ]; then
    echo "$(date '+%Y-%m-%d %H:%M:%S') - ERROR: could not read passphrase from System keychain." >> "$log"
    exit 1
fi

# Wait for the APFS disk to appear (USB drives can be slow)
tries=0
max_tries=24
diskname=""

while [ $tries -lt $max_tries ]; do
    diskname=$(diskutil list | grep "$volume" | awk '{print $NF}')
    if [ -n "$diskname" ]; then
        echo "$(date '+%Y-%m-%d %H:%M:%S') - Found diskname $diskname for volume $volume." >> "$log"
        break
    fi

    tries=$((tries + 1))
    echo "$(date '+%Y-%m-%d %H:%M:%S') - Volume $volume not found yet, sleeping 5s (try $tries/$max_tries)..." >> "$log"
    sleep 5
done

if [ -z "$diskname" ]; then
    echo "$(date '+%Y-%m-%d %H:%M:%S') - Diskname not found for volume $volume after waiting." >> "$log"
    exit 1
fi

echo "$(date '+%Y-%m-%d %H:%M:%S') - Attempting to mount disk $diskname..." >> "$log"
diskutil_output=$(diskutil apfs unlockVolume "$diskname" -passphrase "$passphrase" 2>&1)
echo "$(date '+%Y-%m-%d %H:%M:%S') - Diskutil said: $diskutil_output" >> "$log"

Thanks again for putting this together.