Not a bad start. I assume that you are posting this here for feedback, so here I go. I see a lot of similarities to 971’s frames, which means this frame has the potential to be bullet proof and very nice.
Extend your belly pan all the way to the outer sheetmetal rail, and bend a flange off the bottom of it to make a box structure. This will make your frame a lot stronger. It looks like you might have to move the idler gears up some to make that work, but my experience says that it is well worth the pain.
Put flanges on the 2 vertical faces on both ends of the clamshell. Those will tie in nicely to the frame and add even more strength. See if you can also add a flange on the other angled surface on the outer edge to catch the bellypan.
Battery box? You can nicely integrate that into the belly pan and rear frame rail. You can find some pictures of how we did it (which worked great) if you need inspiration. While you are at it, press in some PEM nuts for all the electronics, and consider punching in some lances to use to tie the wires down with. You can bolt the electronics down with plastic bolts, and then cut them flush with flush cutters.
The row of hole + slots that you have right next to the edge of the frame look nice, but will be a pain to bend, and take a lot of strength out of the frame. I’d either skip them, or drop the slots.
Bend another flange off the inner top edge of the front and back of the frame. This adds even more strength. This frame will hit things, and you need all the strength you can get there. I would also make the flange like 1.5 - 2" long, and not neck down like you have drawn.
See if you can fit some ribs in your box structure to give it more torsional rigidity and transfer the load from the outer rail to the inner rail.
The tolerances on the C-C on the main transmission gear isn’t going to be very good. Bend tolerances are ± 0.010 in my experience, and the stackup on the two bends and the slop in the bolt holes isn’t going to be good enough. We pressed dowel pins into the frame to give us a very accurate alignment, and then kinematically constrained the gearbox from there. Consider holding the output shaft at the end as well. That is a lot of torque to be cantilevering out that far.
What thickness? 090 will be strong. If you are careful and add some ribs, I would be willing to bet that 060 or 080 would work well as well.
Any particular reason that you are using hex shaft for the drive axles? The tolerances on that aren’t so great, which will cause problems with your gear c-c’s again. Same with your idlers.