Sometimes hard engine start

My car has a hard time starting after sitting idle for 6 hours or so, (Cold climate)

I turn the key and it cranks for good 3 seconds before I stop, rest and crank again, it takes about 3-4 tries for the engine to turn over.

Once the engine has been running for a while I can start the car normally until it has been sitting for a while.

Car Idle is smooth, (RPM sits just below 1000 while in Park)

I took the car to a Nissan specialist and he connected the ground wire in the electrical system, and all that did was fix the speedometer and the RPM meter (which had stopped working)

I replaced spark plugs
I replaced starter
Battery charge was measured at around 50% at full charge (being replaced soon)

I took the car to the garage provided by the Nissan Dealership and they had it over night. They said that it was "maybe" the timing chain, and 1 out of 2 Oxygen Sensor was stuck at 0.29v and I was told it needed to be replaced.

They told me they needed to check the condition of the timing chain but I found that too expensive based on them saying it was "maybe" the cause of the problem.

They said nothing about the fuel system, I am not sure they even checked it.

The car runs smoothly, no backfires no misses, no loss in power.

Light gasoline smell after having cranked for 3-4 seconds without results.

Doesn't seem anyone can diagnose my car without a bunch of "ifs and maybes"

You should start by checking your charging system due to the 50% charge of the battery. It shouldn't be that low unless the battery is going ba dor its not charging. Since the car only has a hard time after sitting im suspect of the battery. Charge it fully before checking it, as half charged batteries WILL fail a load test.