Dear Tona,
Definitely choose the salt option for long time storage, you´ll always have a degredation/oxidation of your product when you try to store it as a tincture.
In addition if you have access to a freezer, store your salt in there, so you´ll have a minimum of damage to the precious molecule.
Since you don´t have access to fumaric acid, I can´t help you with the question which kind of acid you should use.
But here is an easy and cheap source for
fumaric acid in europe.
Garfield
“Coincidences are what are left over after you've applied a bad theory.” P.W. Bridgman