Systemctl service with variables show empty - bash

Question

When VM is created, a systemctl service is installed (script1.sh) runs, then later (script2.sh) runs, but variables in service are not updated.
Background
I have a systemctl service which is defined when VM is created. (script1.sh)
Later I collect variables values that I would pass to this script. (script2.sh)
script1.sh
cat <<-EOH > /lib/systemd/system/my_service.service
[Unit]
Description=My Service
StartLimitIntervalSec=600
[Service]
Type=simple
ExecStart=/bin/bash -c '/opt/bin/agent --host=localhost:${PORT} --debug=${_DEBUG}'
Restart=always
[Install]
WantedBy=multi-user.target
EOH
script2.sh
PORT=$(get_random_port)
_DEBUG=$(get_debug_value)
sudo systemctl enable my_service.service
sudo systemctl restart my_service.service
when VM is created, service is installed (script1.sh) runs, then later (script2.sh) runs, but I don't see PORT or _DEBUG getting updated.
Troubleshooting done:
pass set -a
print env and I can see PORT & _DEBUG with correct value
pass Environment to systemctl service.
[Unit]
Description=My Service
StartLimitIntervalSec=600
[Service]
Type=simple
Environment="PORT=${PORT}"
Environment="_DEBUG=${_DEBUG}"
ExecStart=/bin/bash -c '/opt/bin/agent --host=localhost:${PORT} --debug=${_DEBUG}'
Restart=always
[Install]
WantedBy=multi-user.target
I tried to overwrite the service and that works, but does not seem to be a solid solution.