Thanks for sharing ykuga!
I like the idea of disc brakes, but I've found two alternate solutions that I prefer which is to run a DD hub with Regen, and hydralic rim brakes like the Magura HS11. Combined I have all the stopping power I could need on the rear. For the front a rim brake is usually enough as it doesn't need as much force to lock up. That being said, I recently put a disc brake on my BikeE AT at the front due to running a 16" wheel in a 20" fork so had no other option. Works well!
You might want to be careful attaching Aluminium to the steel frame like that. If it gets wet or damp it could start galvanic corrosion. I would ensure you give it a thick coat of paint at least to help prevent that from happening.
As for the battery, yes it certainly looks better having it inside the frame, but it is limited what can be done with the space. I was only able to fit 10S4P in the one above, so 36V 14AH, which is ok for a low powered geared hub, but definitely not enough for what I have in mind with a 2WD fat bike.
I am thinking I might go with 14S8P for this time around for 52V 28Ah (1450Wh). That way I can stack 2 x 4P cells end to end with wire paralleling them to fill up the frame. I should be able to fit 6S (8P) inside the frame and then will put the remaining 8S (8P) either under the sides of rear rack or on top of the frame (or both). Placing the BMS might be tricky, but I'm sure I'll figure something out.
I need to take another look, but it might also be possible to fit 5 or 6 18650 cells length ways into the frame. If that can work it makes a few other possibilities available like 14S10P or 14S12P for some serious long range capabilities.
I'm curious why you changed the forks? Was it a geometry thing?