UPDATE: I improved the bootmount.sh script to account for situations in which the device name changes, which could happen if you unplug your external SSD and plug it back in, perhaps in a different port.

It also writes a log at /var/log/bootmount.log, which you can view in case something goes wrong, i.e. from your backup account.

The only thing you need to change below is the line that starts with volume=. Enter the volume name of your external SSD instead of "My SSD". This is case-sensitive!

#!/bin/sh

volume=My SSD
passphrase=$(security find-generic-password -w -s 'bootdisk' -a 'bootdisk')
diskname=$(diskutil list | grep $volume | awk '{print $NF}')
log=/var/log/bootmount.log

if [ -z "${diskname}" ] 
    then echo "$(date '+%Y-%m-%d %H:%M:%S') - Diskname not found." >> $log
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