Not sure why you want a mid drive, and I'm afraid I don't know of one that would work for you, but I'm sure that those more knowledgable than I will chime in.
Hub drives are great for suburban/city riding. How about a Pedego Stretch, or other cargo bike? Cargo bikes are designed for heavier loads. The Stretch has the battery on the seatpost for centered weight, and the smaller wheels make for good handling. If you have a Pedego dealer near you, you can try one! Pedego offers mag wheels, too, which would be a great option if they have them for the cargo bike. If they don't, you might consider the Pedego Interceptor with the mag wheels. It's a great looking bike. Pedego also has experience with large-sized people in their customer pool, so they should be able to advise you.
I have no relationship to Pedego except as a customer and owner of four of their bikes.
Hi! On the Pedego Owners Group (Facebook), a gal posted who rides a Boomerang with mag wheels. She weighed 450 when she got the bike, and said her weight has presented no problem in riding her bike. If you are interested in her story, prospective Pedego owners can also join that group, where you can find her post. It's a closed group, so I don't want to post further details about her here.
Here's a photo of the Boomerang with mag wheels:
Not a mid-drive, so feel free to ignore my post if it isn't helpful.
It's not a mid-drive, so stop reading if that's a deal breaker, but check out the RadWagon. It lists a max weight capacity of 350# and is outfitted with what I call plus-size tires at 2.3". Rad's website shows the RadWagon back in-stock at 9 a.m. PT on Thursday, August 16th. If you decide to order it, you'll want to have your fingers on your keyboard and credit card in-hand at 9 a.m. PT. If you miss it, you'll see a price increase after August 23rd due to the new tariffs.