I got this Harley Benton DC Junior a few years ago and it's been my main axe since.
Only issue is it has the worse tuning stability of any guitar I've owned, Not only going out of tune but getting it to correct pitch can be spotty in the first place.
Any ideas for replacement tuning machines? I've already replaced the nut with a graphtech one a while back and it only slightly helped when tuning up. I also don't mind drilling new screw holes if need be.
Don't, they're not the problem.
Check some setup videos and see it's not your methodology, check the nut for binding, check that the bridge isn't moving and neck is both cracked etc. And if you've only had problems with one set of strings, try another set and preferably source.