You don't need to reinvent the wheel. There are many many ways to do what you want right now.

I assume you have home internet or Wi-Fi. There are many high quality budget Android phones that can run messaging apps that only require an internet connection.

They don't require cell service. This would be my first choice. If I needed that. Additionally that phone can be used for a 911 emergency call without service if needed.

I may be mistaken, but I think 911 is required by law in the US.

You could use Google voice app that will give you a number you can call that would ring the app on that phone etc. Like I said there are many many ways to do what you're talking about.

Then just leave it on the charger. You can get chargers that won't overfill the battery. Or you could just use a cheap wall charger that cuts the charger off had intervals throughout the day. The battery will stay charged. But won't get overcharged..🙂