but starting this year wont turn over without many tries when colder
Do you mean the engine won't turn over, or it turns over but doesn't start right away.
Let's assume you mean the engine doesn't start right away. You have already eliminated
many faults but I don't see that you checked the batteries or starter.These engines need to turn 600rpm to start. Both batteries and starter need to be in top notch condition. The clue here is an ok start when plugged in and during warmer weather. I had the same problem 2 years ago and like you I eliminated glow plugs, relay,etc. After jump starting and plugging in during cold weather I replaced the batteries. Problem solved. Even if just 1 battery is weak you must replace both of them. This year I noticed starting was getting harder( the engine seemed to be turning over slower). Batteries and alternator checked ok but the starter was weak.Replacing the starter solved the problem.
Conclusion: Make sure all ground connections are clean and tight. Good connections on the positive side also. Batteries and starter need to be in good condition. Alternator needs to be checked. There can be 1 diode out in the rectifier and the alt. will show it's charging but the current won't keep up with the demand this results in the batteries not being fully charged. The glow plugs use a lot of current, not leaving enough to spin the starter fast enough. Sorry for such a long post but I wanted you to understand the reason why a good electrical system is key. Eliminate the simple things first. Hope this works out for you.