1.5 diesel cranks but won’t start

I bought a 1980 westy 4 door with passive restraints and a 1.5 diesel a month or so and am having trouble getting it to start up. Originally it started at the previous owners house with a little help from a jumper box. We towed it home and again got it to start with a jump from my mk4 golf. Drove it into the garage to work on it and that’s where it sits today.
I have done quite I bit of maintenance and testing so far but I am almost out of ideas.
The problem I’m still having is it turns over well for 5-10 seconds before the battery is to drained to turn it over again. Place a trickle charger on it and try again. It acts as if it wants to start but simply won’t.

Wiring:
I have replaced all ground straps with new thicker gauge wires and the starter cable as well.
I have removed the dash and sorted some wires and ground there as well to make sure everything else is working.
I have sorted and placed relays in the correct spot, a few were in incorrect locations. All fuses are correct and unbroken.
I have tried jumping the passive restraint wires as of now I don’t have seatbelts to plug in.
Glow plugs/relay:
I have removed glow plugs and bench tested them and they all glow in 5-6 seconds. I have tested the glow plug relay plate on the fuse box per Bentley and all checks out. According to Bentley I should have “slow glow” plugs and relay but it appears I have “fast glow” plugs and relay. I heard this is not a big deal if you have matching plugs with correct relay but it interests me. Getting 12v at bus bar with minimal to no voltage drop thought out that wiring path.
Injection pump/fuel:
I have replaced all lines with temporary clear lines to check for air and fuel moving. I filled up the IP by pouring diesel into the “out” banjo and into stop solenoid location. I know the pump is full when cranking and I am getting fuel from filter to IP. Bench tested the stop solenoid which looks new and it is working correctly. I am getting 12v with key on.
Starter:
I took out and bench tested it. The plunger deploys and is spinning. No wires look loose or corroded. I also installed a starter heat soak relay as advised in other posts. Not the ford one but the simple 4 pronged relay with a fuse in-line. This seemed to help the starter spin faster and longer but no luck.

I’m stuck at this point. The previous owner stated that the ignition cylinder was new and I checked to make sure it was plugged in good but that’s all I know to test. I also had the newer battery tested at a shop and they stated it was fine but I believe it might be on the way out. Before I go spending money on all new parts wonder where I might go from here. Any and all help would be greatly appreciated. Thanks in advance!