Having read only the first few replies I'll wade in on a different tack (sailing analogy, but sailors don't like having to "wade in").
I had cold starting issues a few years after I bought my tractor, it was GREAT at first but seemed to get a bit worse every two or three years.
I went the lower hose heater route, which helped - a bit.
I went the block heater route, which (with the lower hose heater and a splitter extension cord) helped a LOT - - but I was burning up something like 1500 watts for at least a couple of hours and all too often the need to use the tractor would pass before the block would be warm enough to start anyway.
EVENTUALLY a situation arose where I would have to haul the tractor to help with a project and leave it overnight with NO chance of an electrical outlet within 500 ft.
SO, it was time to figure out what was REALLY wrong with this tractor that cold started so WELL when new.
OK, I've already made this short story into a long one.
I removed all the glow plugs and metered them, 3 were open circuit, one read 38 ohms.
I paid $83.xx for all 4, with shipping - about 1/2 of dealer price.
Probably an hour to get them out and meter them, 3/4 hour to put everything back together.
In retrospect they probably went bad one by one, when the first one went it was so subtle that I probably attributed the difficult starting to the sudden cold spell.
NOTES:
Glow plugs are industry standard items and spec'd by thread, length, wattage, connector type, not_much_else.
So there are interchange tables much like there are for oil, fuel, air filters, spark plugs, etc.
You don't NEED dealer parts for these.
It isn't a HARD job, in fact it is easier than installing a block or lower radiator hose heater.
BTW, that square drive plug IS 1/2 inch, there is NO "Metric" version, it has been a world wide standard for MANY decades - if you are "rounding" it you either have a poor approach angle, a poor 1/2 inch drive, or a lot of crud in there that isn't letting it all the way in.