Same problem man, the fork break bolt are so hard to close and had the same issue with the battery, i'm going with the same solution to sand some material out, even cut the seatpost but it's not enought and hope to not buy another one.

What do you use to sand the step?

I used a piece of thick plastic clothes hanger with a sandpaper taped to it. A more mature solution i guess would be using a round file of length 30 cm or more with a handle. And wrapping it with tape, leaving only around 7 cm exposed. In the process you might try to put the battery in to check if it fits, but be aware not to make it stuck.
In my case cutting the seatpost wasn't necessary.
Also an idea I came up with only after the build - to put a plastic strap on the clip just under seatpost to prevent battery from falling off (image 2).

Good luck!

Hello everyone! I also chose this frame for a new build. It's already finished and didn't went smooth at all.
It all started with a horribly executed steerer tube cut. It is so bad that I want to order a new fork.
First problem with a frameset itself was with the threads for brake caliper on the fork. Tightening required a lot of force, I don't think it should be like this.
Second one was di2 battery not going in the seat tube. There was an obstacle 19.5 cm deep which was called "seatpost inserting the depth security line" by a contact at Spcycle. Apparently I was secured from installing di2. He eventually admitted that it is a problem and promised to improve. After weighting options I decided to remove some material with a sandpaper.  It took a couple of hours to make enough clearance for the battery. It may be done faster, but I used improvised tool which was not very effective. I attached the photo of seat tube made after this process.
Also there was some paint at the edges of rear brake mounts.
During first ride the seatpost slipped. I applied threadlocker to the bolt in the wedge and seems like it helped.

Today I had an 80 km ride. The frame is stiff, maybe even rigid for my liking. Good thing there is space for larger tyres.
Some positives:
Frame looks great, I like thin seatpost and seatstays (they remind of Cervelo R5 from 2010s). The head tube looks better than SL8's I think. And the top of the tube is round and not some proprietary shape. UDH hanger is a good idea. The weight is impressive: 826g for size 56.