How to get to level 40 and beyond in The Witcher 3

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.

All golem type enemies have a chance to drop Golem's Heart, although I personally have only found hearts in earth-type golems, but I'm not enough of a sample size.

In general I would suggest that before you encounter monsters that have rare drops needed for alchemy (Ekkimara, Nekker Warrior, Cave Troll, Cockatrice, Basilisk, and so on) you save your game and reload if you don't get the alchemy drop.