It is GOOD to spread the Airy disk out larger, if the reason is a quality TC, as long as you aren't cropping away part of your composition!
If the lens by itself is sharpest at 600/6.3, then use 1200/13. If it is sharpest at 600/7.1, then use 1200/14. You need the sharpest pupil; not the least pixel-level diffraction.
Most of my longer lenses don't have distance scales, I focus on the screen - when it's sharp there, it will be sharp at the focal plane.
AF doesn't need to be in a continuous/servo mode; with landscape/astro it's typically in single shot mode, and many use the back button focus (so that the camera doesn't focus every time you press the shutter button).
Test shot before the eclipse began
What accounts for the improved sharpness compared to last month? Better autofocusing? Better skill at manual focusing? Better filter?
I'm going to guess either switching from mylar to glass filters or using the auto focus, which worked about 60% of the time.
But I also used a heavier tripod, delayed and remote shutter, focusing cloth so I could see when it was in focus, turned off stabilization, and the atmospheric conditions were slightly better too.
You probably benefitted more from these last than from anything else.
