That Mariabell is the least scary of all villains in this. No significant combat power, and the influence the Rosso have economically or politically is pitiful compared to what Rimuru already accomplished. It was stated that guns won't work on the citizens of Tempest and unless she somehow can create nukes from otherworld knowledge with magic there's nothing they can do that could even remotely be considered a threat. The angels could be a problem but the Rosso don't seem to have any relation to them.
I'm not sure what gave you the impression that Rimuru has more economic or political weight than the Rosso's.
I see that as the area he's currently the weakest in (since he's just establishing his nation officially and doesn't have any underworld ties), he has the potential to get there and has laid the right groundwork but he's nowhere near there yet.
On the other hand they have ties with multiple nations in the east and west and have been doing business for (seemingly) generations, the only thing he got the one up on them with (until this point) was taking over the main trade routes with Dwargon.
Once he's able to establish official trade routes with other nations that's when he's going to outstrip them (and that's what they're afraid of).
So yeah while it's not direct (or rather BECAUSE it's not direct) I'd say they're unironically the toughest opponents he'll have to face for a while.