There are Mastodon instances with hundreds of thousands of users and millions of messages.
However, Mastodon is federated, so there are thousands of instances with less users. And because each instance does not need to store the state of the whole network (only accounts followed by its users), it scales very nicely.
I feel like that's the answer to reddit it many ways too.
Reddit will go the way of Facebook within 5 years I suspect and self hosted federated subreddits are the logical increment. I bet someone is working on it already.
I plan to use ActivityPub so it can integrate with mastodon and peertube more easily but also have an option to pull over from reddit so people can use their own instance as a frontend to reddit. Makes migration easier IMO.
Your profile has no links, it there any page to follow what you're doing, or are you developing it offline? Anyways, I'd love to be able to follow news about what you're on to, it sounds really exciting!
However, Mastodon is federated, so there are thousands of instances with less users. And because each instance does not need to store the state of the whole network (only accounts followed by its users), it scales very nicely.