I thought Telegram does encrypt chats, just that you have to enable it rather than it being on be default. Is encryption only available for one-on-one messages?

It's only available for 1-to-1, yes. But why would you rely on a service that hides the option to E2E encryption so well? Next step is to silently remove it.

Signal is private by default.

It does not have the option to encrypt group chats last I checked, and even the one-to-one encryption is not particularly well-liked among security experts.

This isn't about casual chats with friends and family, but political activism against the actions of a country. People doing that should be willing to put at least a trivial amount of effort into security.

Telegram is owned by Russian Elon Musk and the FSB. Avoid it if you can.

Love my mom. When I was experimenting with the Pinephone and was only textable via SMS she begged me to get something that could run Signal.

Generally, the issue is that services want to blend the data into a single stream, otherwise you end up with having to make a separate connection to and from each client, each at full bandwidth called full mesh. It becomes easier to just have the encryption to the server, then decrypt all the streams from all the participating clients, and merge them into a single stream outbound. Adjust and re-encrypt that single stream and send it. That also allows for more control to make low bandwidth or dynamically changing bandwidth clients work better as you can adjust one stream rather than hundreds of outbound to each client. But that means the server has access to unencrypted streams to analyze and record. This is called server mixing.

Previous software generally only supported one or the other of those options. Signal is one of the few that have developed the technology to allow for selective forwarding which is the third option. Their website details how this works and it's open source so some others have adopted it or have come up with similar approaches.