"Just move to Matrix, Discord is insecure”
I mean, yes*. But also: the community will cease to exist if you try to move it. Communication technology is situated. Community is in its connections, and if you bifurcate it, it becomes two different communities.
If you wholesale shut down the old and start the new, you'll have a very different community when it arrives.
Maybe, if you get a bulk of people together and organized, you can pull it off. It means the slow careful work of mapping enough relationships so that you know you have a coherent cluster of people aligned to do the move.
* I have a whole rant about what "secure" means in the context of community but you'll have to wait or ask for that.