Fediverse

Matrix actually supports multimedia, GIFs, videos, and photos. The speed of downloading multimedia always depends on the internet connection, the available host machine, and the computational resources available to the host machine. This obviously applies to every hosted platform.

In terms of privacy, Matrix has encrypted rooms, which means that the actual messages, the actual text of the messages, will be encrypted and stored on the instance in encrypted form. The only way to decrypt that data is to use user keys, or client keys. So, in short, it is a more comprehensive security system. It can theoretically be compromised, but it would require more effort to do this. So, in terms of security, Matrix is really good. If your priority is to keep your friends and yourself safe, which I think should be the first priority, then Matrix is a very good solution if you eventually end up self-hosting.

Regarding other platforms, Mastodon actually supports hiding data and restricting some API calls in order to make the instance more closed. But on the other hand, the data is stored unencrypted. So if someone gets access to your machine, to your host machine, it will be pretty easy and simple to get and read all the data. Again, in comparison with Matrix, even if someone eventually gets access to your host machine and to the data, it will be another challenge to decipher the data they have.

Speaking about forums, another option is the NodeBB forum platform. It is also federated. It also allows you to hide the data from unregistered users. It can manage roles for users to make certain users see certain categories, so the management system is more comprehensive in this sense. It can also be federated, so categories are federated there. So, in some sense, you can have a federated closed forum using NodeBB.

There are also other platforms, for example, Hubzilla and Bonfire. They are more privacy-focused federated platforms. But as far as I understand, they are more like social platforms instead of forums or microblogging platforms. So they are more like Facebook in some sense. They have some specifics.

So, in general, I would recommend using Matrix if you end up self-hosting and if your main priority is security. Again, it supports multimedia. But if you still want a more forum-like platform, I would recommend using NodeBB, or taking a look at Hubzilla and Bonfire.