Yes, very low network effect though, that might not be a problem.
But real problem is absence of desktop that synchronizes with mobile. SimpleX has a desktop version, but it has a different set of chat profiles and you can't sync unread status.
Element got pretty cool lately, especially with spaces. I don't do 15k rooms though. If there are 15k people, I think encryption kind of stops making sense.
Here's my encrypted messengers blog, I try to keep it up to date.
https://juraj.bednar.io/en/blog-en/2022/05/03/encrypted-messengers-comparison/amp/