Exported my vault, erased all content in /data, recreated the container, recreated my account, re-imported my vault. ADMIN_TOKEN works and I can create settings.
If I recall right, if you just remove the line and restart the container the admin page will be disabled. I would just have a separate file in the config directory where you remove it from the config.json and paste it into something like unused-admin.json (or whatever- doesn’t matter.) That way you can just copy and paste it again and then restart the container when you do want the admin page available.
Depending on reverse proxy, you could also just limit bitwarden.mydomain.com/admin to only LAN IPs. Or only your specific desktop IP, etc. etc. etc.