After posting my question, I ended up mounting the rack and fender to the same behind-axle bus, which levels out the rack.

However, I didn't think about the shear stress and mounted the fender eyelet in between the bus and rack, so I'll need to undo that and swap their positions.

It's a bit crowded and I'm worried about the rack eyelet corner hitting the axle. The center rack brace also blocks the lever that allows removing the wheel. But if I bring a toolkit with Allen wrenches on trips, I should be okay, I guess.

I definitely did not want to DIY a solution that ends up not supporting the weight or ends up causing a component to fall into the wheel while I pedal, so this seems to be a much better solution.

Thanks for all the tips! Want to make sure I do this right the first time.