I've long known of Yasha Levine's criticism of Signal: it was funded by the US Government who needs encryption for it's own spy apps, and it needs other people to use them (!!!) or it's obvious that everyone on them is spies.
So, we got Tor, and also Signal.
That doesn't necessarily mean they're bad, does it?
But signal has other issues. The use of phone number identifiers, the centralization, and the laws of the USA which prohibit any company revealing that they provide info to spy agencies, which you can assume that all telecommunications companies do (and I learned about this years before Snowden, but Snowden confirmed that it was indeed actually true).
So the US government axiomatically has access to all the metadata, the who contacted whom and when. Which is all they generally need or use.
Here's an article laying out other options, which would include such things as self-hosting (which Signal manages to block). Top of the list (and I've seen this elsewhere) is Matrix (client name Element), whose identifiers are as secure and non-linkable as you want to make them (so do), and comes with Jitsi a free self hosted video teleconference program. Briar, XMPP, and SimpleX are interesting alternatives.
I'd be interested in these things if I were organizing. Otherwise, I simply assume I'm always being watched. So with that assumption, Signal is fine, heck, even Facebook is fine. I live electronically, I'm not willing to give that up, and I know what it means. But I don't much care.
There is of course the fact that if you use any internet protocol, the US government will know where it came from and where it went. They have people listening to the radio too. And the more effort you try to put into staying under cover, the more it may be obvious that you are doing so, and raise suspicion for that reason alone.
If I knew the best solution I wouldn't say. Perhaps resign yourself to futility. No resistance organization will ever be capable of bringing down the empire. The best the resistance organization could be is pick up the pieces when it falls apart. (That's the way I see it as a US citizen. It may be different as a Palestinian, etc.)
That's what Lenin did, and even with his organized cadres it was pretty difficult to keep it from getting subverted, as ultimately happened with Yeltsin.