Given what you've described, I would guess that your substrate is too thick. Try using just two sheets of newspaper or newsprint...OR...just a very thin layer of aspen. They don't need it for cushioning...just to absorb waste when they make a mess.

I'd say 105* is at the high end of tolerable, and hopefully, with less substrate, you can get that down another few degrees while maintaining the temps you want. Just make sure that the actual floor of the tub is at the temps you want and not sitting at 100+ degrees.

I think you're really close to getting everything dialed in just right.