iMessage on MacBook Pro intermittently fails to send messages and sign in

About once a week iMessage on my mac will just stop working - like it’s open but I can’t send messages of any kind to anyone - iMessage users or non-iMessage users. (it'll just say "Not Delivered" with the exclamation point in a circle icon.) iMessage continues to work fine on my iPhone (13 mini) and iPad Air (2025).

I’ll sign out of iMessage on my computer. And then when I try to sign back in, it'll load for a few minutes and then in red the message "An unknown error occurred." will appear. In this state occasionally messages randomly come through and/or old messages will get sent but if I try to send something myself right after it'll still fail.

So I’ll restart my computer. After this, multiple things can happen. Either:

1. I still won’t be able to log in. I usually abandon my efforts at this point and wait a few days and try again. (Eventually it'll work.)
2. I can log in but still can’t send messages.
3. I can log in and send messages but only to iMessage users.

The functionality of sending messages to non-iMessage users doesn’t usually come back right away but it does with time. 

I am on a 2019 13-inch Macbook Pro with the latest possible update installed. (It doesn’t support the most current OS.)