Win10 Setup - "System Volume has no space"

Hello,

I can't seem to continue on with my Windows 10 installation. When I click on 'new' nothing happens. There were a few attempts before where random errors with different hex codes would pop up while doing it.

I found a solution involving formatting the EFI partition during setup but I'm reluctant to do that. I tried mounting the EFI partition back in MacOS and removing about 150mb of cache files but then I noticed that even finder reports no available space on it.

Extra info:

1. I'm using a USB 3.0 thumbdrive with the setup files + bootcamp drivers at root. I couldn't use BC Assistant because it always messed up my autoAttend.xml file when it rebooted to setup.
2. I managed to copy OSXReserved to that thumbdrive before my mac restarted when I used BC before this.
3. Modified the autoattend.xml at root: "<DiskConfiguration><WillShowUI>Always"
4. gdisk reports MBR protective with GPT present.
5. The unallocated space was previously MacOS Journaled partition. Formatting it to fat-32 makes the disk MBR hybrid and win10 setup wasn't too happy about that.

Apologies if this isn't the right community to ask.