I’ve been testing some ultra wide newbee hubs. The internal width is 92 mm. It is the dm1 with a 3d printed spacer between. They’ve been fine so far for me but I haven’t had the time to test them further. Lmk if you’re interested and we can chat!
The widest off-the-shelf wheel I know of is Trampa’s Deep Dish Megastar 9.
I’m pretty sure the width is 3" if measured from the outsides of the flanges. I’m using them with 8x3 kart tires, and the combo works pretty well, but they still have a much more rounded profile than I was hoping for.
They might work with 3.5 tires if you can tolerate some sidewall bulge, or fabricate a spacer to widen them a bit further.