Howdy.
I would recommend checking and cleaning the contacts first. Its quick and easy and a common cause for problems like this.
However it might be the Tailcap. Ive had a similar problem happen before where the tailcap poops out and becomes really unreliable. I can’t think of a good way to get a new one other than buying a new light unfortunately.
I thought a C10 was a C8 with a deeper reflector. Unsure how it work be more compact or fit better?
As for batteries, some are certainly better than others. However, I wouldn’t expect huge gains to be had here. Maybe +/- 20%? You can do some searches for “18650 best” and probably find a lot of info.
I would recommend a flood to throw XM-L light, if you can find someone to customize it for you. Most of them are lousy because they are underdriven. If you can put a good driver into an XM-L with the modes you want, then it should work pretty well.
Your flood will look brighter because you won’t have a glaring hotspot…I way prefer this for working or lighting up an area because its even lighting.
You can adjust to throw and it still works decently.
I have a post that compares my XM-L2 F2T with my TK70 which is a beast of a light…and it does decently! Zoomie Comparison: XM-L vs XR-E (pics)