That would probably be too close to the alchemist.
The Herbalist is supposed to only cover a small part of what the alchemist can do, just like the Poisoner archetype.
Personally, I think your best bet is to go Alchemist multiclass. Blave is right; the kind of modification to the Herbalist you're looking for strongly overlaps with what the MC archetype is already doing.
Flavor wise though, I think you're right that it fits well. And because of that, it is entirely possible they'll open things up down the road. I could see a 6th or 8th level feat opening up your herbalist alchemy benefits to all elixirs, including mutagens, though without increasing your advanced alchemy level via the Expert Herbalism feat, you'd have access without being able to make the higher level ones.
There's also the possibility of a completely separate archetype that is focused on mutagens. Perhaps something will be in one of the 2021 books.
My fingers are crossed for an equipment focused book late next year, but yes it would also fit in well in the Mwagni, or even in the Strength of 10000 AP (I believe the academy has notable herbalist-alchemists).