Solution !
Just posting solution in case anyone in the future stumbles across this thread.
I've managed to fix the problem by doing the following (was just trying anything really)
1) Download stock ROM
2) Flash stock ROM, Bootloader, CSC, recovery
3) Re-flash TWRP - Now able to load TWRP again !!
4) Format Data partition WITHOUT using the swipe action (this was in the original instructions, and gets rid of Samsungs encryption. RTFM I guess? )
5) Whilst in Recovery, copied Lineage OS ZIP to phone. Installed, and reboot.
6) Now able to use phone with Lineage OS !! Stoked
