Patreon plugin does that. Every single patron is in a catch-all default “Patron” group, plus a group for each tier.
That is very different from what the Patreon plugin does.
On the Discourse Patreon Integration we have a clear separation of concerns. Everything payment in on Patreon and Discourse just reads from Patreon remotely. There is no user UI in Discourse about Patreon at all.
This deeper level of integration means it would be a lot more work than the Patreon one. Also it makes it very similar to what Discourse Subscriptions plugin does. Did you take a look at it? Maybe Steady plugin could be a fork of that one instead? Or an alternative payment provider inside the same plugin?