In my experience OpenVPN drains the battery like crazy. Personally I have port 443 and 80 open and use a reverse proxy for everything I need accessible outside. Though if you want a vpn that doesn’t drain as much battery and connects almost instantly I could recommend WireGuard. I have been really impressed with its performance and battery usage. I can litterally leave the VPN connected all day without taking a big hit on battery. There is also a vpn on demand service though as I have a server that I connect to remotely without vpn I do not have any experience with that.
If you use Hass.io you can simply download the plugin. All you need to do is open the port and add a user.