You can’t have it both ways.

Of course I can. Jitsi Meet lets you do it both ways. I don't know if Nextcloud has an official hosted server but they could if they wanted. I use it self-hosted and it works, the Talk app is just not very good. Jami uses a DHT instead of a centralized server which is another approach, though it might be part of its flakiness. Linphone (a regular VOIP client, not a secure chat thing) is set up by default to point to Linphone's own SIP servers but you can change that in Settings. No reason Signal can't do similar. Heck, even Lemmy works that way (you choose your server).

Signal is simply being evil and your defending them is unconvincing. I could opt to self-host Signal and build a special client for my users, at the cost of hassle for everyone but no serious technical drawbacks. Signal chooses to create that hassle because they want to funnel users through their servers, not incidentally collecting metadata about ALL the user conversations.

There's actually a configurable Signal client called Amanda or something like that, though I haven't tried it. Someone here mentioned it last time this came up.

Also, Signal's own client isn't on F-droid, which raises more potential questions. I haven't cared enough to look into it.

Added: oh re Nextcloud, I see what you mean, account creation is an obstacle, though that could be handled like Hipchat used to. You could generate a randomized URL to invite someone to your private chat without their needing an account. Nextcloud has that too, though just for file access, not for chat for some reason. Come to think of it, Signal could also work that way: it shouldn't need accounts at all.

When I've invited people to my Nextcloud I've just enrolled the account for them myself and told them "please log in with username X password Y".