Update again, after much searching and discussion with UK group chat.
One of the members pointed out my misfiring, loss of power etc could be driven by the master cutoff switch.
For giggles, I bought a new one.
See the below video, but the relays in the original switch are lose and wiggling.
I have the vehicle on a tender and will hopefully get a start sometime this week.
If this is the cause of the issue I have been chasing for over a year, I will come back and update this post with key search terms for someone else who may be struggling with this.