Starmaker: The best metroidvania on GOG is currently
La-Mulana. It has a couple pants-shittingly scary areas but (despite dealing with cosmic horrors) is generally colorful and cheerful.
I should probably warn you that this game's puzzle difficulty is very high, with some puzzles that you will probably not be able to solve without looking up the solution.
(Interestingly enough, if you do look up the solution, it can still be fun to figure out which clues lead to the solution and how.)
I could mention that Guacamelee is a metroidvania, though I note that it focuses on melee combat (the only ranged attack you get is to throw enemies at other enemies), and that you *need* a controller in order to play the game comfortably. The game requires lots of precise inputs in order to get past platforming sections, and there's no guarantee that any given keyboard can handle that many inputs at once.
You could also try Dust: An Elysian Tail. There are towns in that game, but many of the areas actually give you that alone feeling, and there is one chapter (the longest one, I believe) that has a Castlevania-like horror feel.
If you want a non-violent game that gives you the feeling of being alone and solving puzzles, how about
Waking Mars?