Matrix would be my own, personal suggestion as well. I didn't name names in my original comment because I didn't want anyone to feel pigeonholed into something, the most important thing right now is simply getting yourselves safe if you need to. But yes, if you want a suggestion, Matrix is probably the best idea for all the reasons above. Plus, you are free to use any Matrix supported client, I admit pickings are a little slim in terms of variety on the front of clients atm, there are a few but they are functionally largely identical currently. But hopefully that will change in time, faster if the standard gets more popular.