I've tried all that and more. I'm 48 so have had decades to narrow it down. None of that, over extended trial periods, has made any difference. 5000iu Vitamin D is the one that seems to be doing it. Also taking Vit K as well so it may or may not play into it.Need folic, zinc with lysine. For bad cases eating raw garlic or taking black garlic (not as good) everyday can help. If you don’t mind pain there are liquid garlic supplements that can be applied to the sore.
Avoid arginine and nuts, chocolate, seeds, oats, generally avoid plant oils with high omega 6, but animal, coconut , sesame, fish are fine.
Ideally combined with a low carb diet.
Xylitol and mannose may also help .
Lysine works in much the same way as xylitol/mannose so If you just take lysine but are overloaded with the things I mentioned it won’t help.
Good idea to get a full panel of allergens and intolerances. You may need to go on the elimination diet / liquid to see if you have leaky gut/intenstional issues.
When I eat foods I am allergic to I get gut issues followed by skin issues like cold sores.
It's so different for different people - a tough one. Your advice would help many, no doubt. Hopefully they figure out a vaccine sometime....