Experience points for solved low level sidequests

The main story quest on Skellige is marked as level 16, and it is not particularly difficult combat-wise. You should have no issue doing it, and it'll give you a nice bunch of XP.

There are some other quests on Skellige around level 16-20, so there should be enough to do there. You are more likely to accidentally run into enemies that far outlevel you on Skellige, so you have to be a bit more careful.

Equipment can matter a lot, so if you're having a hard time make sure you have the appropriate witcher set items for your level, or other good gear.

I did switch around Skellige and Velen/Novigrad between levels 15 and 19, there is content on both maps for these levels. I'd recommend to just try out the main quest on Skellige, it should help you level up and you'll find a few more quests for your level on the way. The main quests grant a lot of experience, more than anything else you can do.

It is absolutely worth it to craft one of the Witcher sets of armor, either the feline, griffin or ursine armor. Those are pretty much the best armor for their level and can be upgraded three times. You can buy maps that will lead you to plans for witcher armor at various merchants.

Other armor pieces are probably not worth it unless they're a significant upgrade and you have the materials available. Buying materials can get extremely expensive very quickly, but you can find almost everything lying around somewhere.

The Griffin armor is a medium armor set that requires level 8, if you want to craft something now I'd suggest you try to find it. The feline/cat armor is level 14, and the bear/ursine is level 17.