You can change the replies per day on a trust level basis already in site settings. However, you can’t control how many topics a user can open out of the box. There are create topic / post rate limits but these apply to all users. [1]
I would suggest:
- Using groups to control who can
create/reply/see
on certain categories. - Have a look at the Discourse Subscriptions plugin
- Have a look at the Discourse Patreon Plugin
- For a custom solution post your requirements in the marketplace category