When you say "smoke test" what was done? If you meant an MOT smoke test that would be with a warm engine and not really tell you about your problem from cold.
Another garage smoke test is introducing smoke to the turbo inlet to check for leaks in the inlet system, this is largely irrelevant when starting an engine.
The one I was referring to was standing at the tail pipe of the exhaust when engine is started from cold to see if white smoke is visible often combined with a smell of diesel.
It is caused by unburnt fuel and can be faulty heater plugs etc. which you mention have been dealt with, but also can be caused by low compression on one or more cylinders of the engine, the other name for a diesel engine is CI which stands for compression ignition, so in simple terms if you have low compression you have poor or no ignition
When injectors were out I would have done a compression test as easy then. This would have confirmed or eliminated that side.
Once engine is warmed up then the fuel burns easier and so problem less obvious.
Another indicator of low compression if caused by piston/bore wear can be higher oil consumption, but not always.
