Hard starting can have several causes: low compression
, low ambient temperature (or bad glow plug) and poor injector pattern. If one of the three is really bad, it will be difficult to start. If two or three are marginally bad, it can also be difficult to start.
In my case I have a cat with two Yanmar
3gm30f engines. I had exactly the same symptom as you describe. Although that engine doesn't have a glow plug, low ambient temps made it harder to start.
I replaced the injectors to improve one of the three factors and it made a huge difference. After a few hundred hours the hard starting returned, but running the engine at high rpm
for a few hours cleaned the injectors up and it starts easily now.
So look at your glow plugs and your injectors, two things that you can easily fix.
Low compression manifests itself with blowby that can be easily checked. But fixing it is expensive.