Hello Jag forums, browsing this site has been an invaluable resource in what I've done so far, and I think its time for me to ask for my own help.

In November of 2022 I purchased a 1986 XJ6 Vanden Plas, and drove it about 240 miles home with no issues, save for some scary moments on account of the failed brake master cylinder/booster. The previous owner instructed me that only the left gas tank was functional, but couldn't say why, but he did replace the right gas tank and that was brand new. Whatever, I'd figure it out when I got home. I purchased the car with the intention of keeping it forever. Despite reliability warnings from American car enthusiasts in my life, I was aware of their reputation for needing more care and maintenance, and was/still am happy to put the extra effort in for a car I truly love.

Things I and my ex-American car mechanic father have done to it at first:
-New Valve Cover Gaskets (There weren't even old ones, just terrifying RTV sealant, but at least I was incredibly impressed with the condition of the valves)
-New Brake Master Cylinder
-New Brake Booster
-New Power Steering Rack
-New Right tank Fuel filler gasket (Also RTV sealant at time of purchase)
-Right tank Fuel Filler
-Replaced Aftermarket radio with a newer model
-Fixed bulb/relays for left side indicator lights
-Repaired/Created new wiring/relays to create functional headlights/high beams
-Completely custom Exhaust headers (including appropriate O2 sensor bung and catalytic converter delete, as I was unable to find fair condition manifolds, resulting in 4 months of waiting/ car sitting, still has a medium exhaust leak near where it connects to the stock system)
-Repaired horn mechanism inside of steering wheel
-New wood veneer on center console (Old was cracked from 4 decades of the sun)
-New Speedometer/Odometer combo (Previous odometer was stuck at 120000)
-Reattached rear window Brake light

For a while after the custom headers and the motor could run, we discovered it had an issue with dying when put into gear. Just cutting out. This led us onto a path of diagnosing more problems with it, and replacing faulty/less-than-desirable condition parts.
-Coil
-GM ignition switch module inside the ignition amplifier
-Spark plugs
-Spark plug wires (old ones had been chewed on)
-distributor cap and rotor
-ballast resistor + wiring
-coolant temperature sensor
-ignition switch
One of these made the car lose spark, but not before we discovered another issue with gasoline coming out of the tanks when the motor did run in park. This led to more tests and replacements along the way, including
-new fuel injectors
-fuel pressure regulator
-successful fuel pressure test
-no noticeable vaccum leaks after running a smoke test
-new air filter

After all of this and the car not having spark, I decided to take it to the nearest antique Jaguar specialist to at least figure out what stole my spark. 8 hours later they discovered the distributior air gap to be incorrectly set, diagnosed and repaired a wiring issue from the control module for a fuel injector pulse, and got the car running but "runs poorly and ran out of fuel". Apparrently on putting gas in each tank it came right out, even with the motor turned off, unlike with me. Another 8 hours later they had replaced the right tanks return valve and correctly wired the changeover valve (something new the previous owner installed incorrectly). Upon picking it up I learned that the timing may have been advanced in order to have it start That was 2 days ago, as of today it does start with a jump, runs incredibly rough for between 10-40 seconds, and cuts out completely. They offered to keep it to try and figure out what was wrong, and while I have no doubt they could, with the disappointing combination of 150/hr labor rates, invoices about as descriptive as one sentence per hours of work, inability to put the car back together after working on it (the interior trunk panels were strewn all over the place, and I still have to go back to get my air filter they forgot to put back in), I can't afford to let other people work on my car.

I'd love to have a day locked in my garage methodically repairing and testing, but I'm not too sure of what to do. I did do some research on ground straps, and have heard of their importance. I've cleaned a few grounds throughout this process, but I checked the thick ground strap underneath the car on the passenger side, from the motor to the body, and upon touching it layers of dirt fell off and my fingers were black. If nothing else, I'm sure cleaning that up is a start, but I doubt it alone would be responsible for so many engine running issues. If it's that dirty now, it was just as dirty when I drove it 240 miles with no issues 9 months ago. I imagine I'm one of the younger/more inexperienced members here, but I'm determined to put as much time/effort as necessary into this. No one in my life understands why I'm so dedicated to this expensive car as a broke college student, but I'm certain people here do. I deeply appreciate any advice or insight. I only brought it to the specialists due to my knowledge running out, but I know this site has the collective knowledge to solve just about anything.
I might post a video of the engine running after I take my air filter back for some extra context/information, I think it could be helpful.
Thank you