Cold Start issues

We've got an issue with our '94 Hiace and are hoping to get some help.

Engine is 3.0l 1KZTE
Using Diesel winterizer in every tank
occasional fuel injector cleaner
Typically have Oil pan heater plugged in for an hour or so before starting.
Elevation ~6000ft

Starting the engine in even moderate cold temps ( <40F) is very challenging.
The engine will turn over many times before eventually starting.
It will idle fine for a while, but after 10-15 minutes it will start to shudder and make "thud-thud" sounds intermittently. This happens about the time the temp gauge has lifted off the bottom.
If you give it a little throttle through this phase, it will usually keep running.
If you try to drive it here, it will die and not be able to restart, sometimes for hours.
The car will be good to drive after about 45 minutes when the temp is solidly in the center of the gauge.

Of note, having the nose of the car uphill will make this situation much worse and instantly kill the car (or prevent it from starting in the first place). If you start driving after only 30 minutes of warming, and encounter a slight uphill, the engine dies and wont restart until you get it to flat ground and wait a half hour.

Any suggestions of what the issue may be, or how to further diagnose the problem would be greatly appeciated.

It sounds like you may have multiple problems , the hard to start one is either heater plug fault or fuel supply problem , the juddering after 10/15 min could be a temp sensor or ECU fault . The head up issue points to fuel supply issue . Start by testing for air in fuel supply system , test the heater plugs and system by a test light on said plugs light should light up for 90sec with audible click from relay when switching off. Fuel supply has four screens or filters that do get obstructed ours all did plus unreliable filter head pump , the lift pump is in the IP and is pulling fuel all the way from the tank thus pulling air at any faulty joints etc . If all is good it could be the capacitors on the ECU that is a whole can of worms as we found out but not incurable. Good luck.

I has an issue before with the van not restarting when warm - it had to cool down a fair bit before it would. It was the petrol version but it turned out to the the 2-wire temp sensor for the ECU (not the 1-wire sensor for the temp gauge). It's a quick cheap fix to replace - just do it when the engine has been sitting for a good while so it's completely cold - so there's no pressure in the cooling system and you won't loose any coolant other than a few drops. And make sure the new sensor comes with a new copper sealing washer - the old one will have been crushed and won't come off the old sensor. Most after market sensors will come with the washer, but official Toyota spares won't - so you have to order it separately!