Just try the 16x2.25 Moped tire above. No guarantees, but they might work if they fit the frame.

If you're wanting to make a custom wheel, maybe try the Sun Big ******.
Sun Big ****** 20" 36h All Black rim, Schrader valve, 32.5mm width

Of course, that may be a lot of work for increasing tire life. But, if it also saves you a few flats and headaches, it might be worth it.

I think I mentioned earlier the Tannus line.

Tannus Thoroki
20x1.5 (40-406). $55.
https://squareup.com/market/metro-gn...ead=item_embed

Looks like this one has soft and hard options: I'd probably go with hard for your cargo use. Plus, only $29.
https://squareup.com/market/metro-gn...ead=item_embed

The Tannus tires should last you quite some time, depending on how far you wish to grind them down, and no flats.

I'm still a few days out on testing my 700c solid tires