Watched your video and I had a headache watching it but i do like our enthusiasm trying to get it street legal. I can understand why no one wants to take on this job immediately/asap. You are not wrong either as most shops can do a rebuild with far less effort than try to figure out the wiring mess in your car. Try Tim at Rx Motors, he miraculously fixed my electrical abs issues which many pro shops were unable to fix.
option 1 (cheapest) - who did this job? take it back to them and beg to get it road worthy

as they are most likely the only few who will be able to understand how to integrate stuff into this harness or why it was even done this way?!likely the cheapest option. Although you've spent over £700 without any results so far!
option 2 (not cheap) - start fresh, new link g4x ecu, fuel tech / modern ecu of your choice etc and new custom loom. I doubt any reputable tuner will be able to do this work for cheap because its technical. New ECU £1-2k, £1.5k for new loom and up to 1k for a full map depending on who does it. Plus another 1k for digi dash - this could cost anywhere from £3.5k+ (If money is no object then contact
https://www.renvale.com/ they are probably one of the very few tech experts who might be able to build a loom etc with your current setup....assuming they take on the job.)
option 3 - learn to build custom looms and how to map with hp academy and it all comes down to how quickly you can grasp/apply it to the problems you have.