Big Sur setup halts on language selection - Intel NUC Skylake

I've been trying to get this to work for weeks but cannot get beyond the screen where you select the regional settings.

From what I can tell the basic install/download happens after I create the partition (this all works fine), I get 4 reboots to the Apple logo with the progress bar (29 mins remaining, 16 mins remaining etc) and then when the setup begins I get the select language screen and the PC reboots.

The PC reboots shortly after loading the screen (<10 seconds). Once I was quick enough to select the language and choose continue, but most times I get half way scrolling down to United Kingdom and the PC reboots. this makes me think it's either a CPU management issue or a graphics issue, but honestly I have no idea. Some hints or things to try would be greatly appreciated.

Hardware:

Intel NUC6 - NUC6i5SYB

• CPU: Skylake Intel Core i5 6260U
• GPU: Intel Iris 540 - Using an ultrawide connected to HDMI - 1440 x 3440
• RAM: 16GB (2x 8GB 2133MHz)
• Disk: Samsung 950 Pro 256GB, single partition. Don't intend to install any other OS
• Motherboard/Laptop model: NUC6i5SYB
• Audio Codec: Intel Integrated Sunrise Point-LP HD
• Ethernet Card: Intel I219
• Wifi/BT Card: (if available) Don't intend to use
• What guide/tool followed: Dortania OpenCore Guide
• What files/config I am using: Tried with OpenCore 0.6.3 and 0.6.4. I have tried both the pre-built SDDTs referenced in the OpenCore guide and building my own using SDDTTime (both desktop and laptop).
  • ACPI: SSDT-EC, SSDT-HPET, SSDT-PLUG, SSDT-PMC, SSDT-USB-Reset
  • Kexts: AppleALC, IntelMausi, Lilu, VirtualSMC, WhateverGreen
• Extra Info: I'm using iMac 17,1 as system type. I have tried other configs as I know some people say for Intel NUCs to use laptop components but when I tried various MacBook versions I didn't even get to boot. I have tried following the Dortania guide for both desktop and laptop Skylake based computers and get broadly the same result.

Hope this is enough for someone to help. From googling around, this does not seem to be a typical place people get stuck with the installation, so I don't really know where to go from here.