If you really need to stick with a single speed, you will need a small cog in front and a large on in the rear. Top speeds will be highly limited.
@PedalUma did a better job of explaining what I was trying to get at with regard to gearing down for low speed.
@rich c came at it well, also. Your problem is that single speed and what that means when it comes to hills, and starting from a stop on flat land. Ordinarily the whole point of a mid drive is it gives you the ability to work thru the gears to help you get up hills. You typically have to downshift it so it doesn't bog and kill your drivetrain from a standing start. You have to counteract this limitation, and the way to do it is to just stay in a low gear in the first place. That means a little front chainring (Lekkie makes a 36T wheel that I use on one of my cargo bikes) and the biggest single rear cog you can get your hands on. The hill-climbing cargo bike I have that 36T on in front has a 42T in back (it goes to 46 but I can't reach that one thanks to chain line and super fat tires).
If you want to blow your budget all to hell, but REALLY solve your problem, put a front hub motor on for a 2wd solution. I have done several of these and they are positively stellar in hills under load. My flat-land Bullitt is set up like that and I am building another one for hills right now that will do the same thing, but with drivetrain set up for low speed and hi torque, because steep hills.
The front motor kicks in immediately and starts doing work, so the mid drive, which is set to engage at about 5 mph, engages an already-moving bike and doesn't have to kill itself and your chain starting from a stop. the results are amazing. On my first one of these, I did not downshift at a stop, and starting the bike on my 11T rear cog, that cog lasted for 1000 miles before I finally cracked it. Ordinarily if I was doing that you could expect it to die within 50-100 miles, tops.
@Bigjayvapes seriously you are well into the territory where a 2wd solution - 3wd in your case - makes a lot of practical sense. Honestly as I said earlier, making this happen with a single-speed is a tall order. I don't know where you are going to get a big enough rear cog for this. White Industries is where I'd go for one and the biggest they make is 22T. Thats well underneath what I'd be comfortable with and a second motor helping out might be the ticket to make it work (OR give in and put a derailleur on... which has to be cheaper, easier conversion than going 2wd). Give it some thought. Here are some details on the concept. There are fairly detailed build writeups here as well.
I like to build top-quality-component ebikes from the frame up. Quite a few of them are dual motor or AWD or 2WD or whatever you want to call them. Why would you build an AWD ebike?
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