With Linux, you need to have the awareness of what is running on your phone 24/7 and AFAIK, there’s nothing like that except maybe Home Assistant, but that’s pushing it. I have a buddy which has an always on VPN which he uses 100% of the time at home, never any issues with him. As for the phone, I have Wireguard for when I am not connected to my WiFi, and then disconnect when I’m back on my WiFi network. This way, I am certain to be on my home network 99.99% of the time. I use Tasker for that.
With that said, you will need an automation of sorts on the Android to be 100% sure you are on your appropriate VPN. Whether through Tasker, Macrodroid or a few other apps.