First of all, I can't seem to figure out operations on existing data, it allows me to specify that I want to use operation but not which one (see first screenshot)
Second, it would be great to have access to string storage from /execute store, such as /execute store result storageplayer @s path string

/data modify storageplayer @s storage operation (=, +=, -=, *=, /=, %=, ^=, <, > ) in this context operation means Math operation, numbers can not be concatenated.
To concatenate strings use /data modify storageplayer @s testOut set concat storageplayer @s testIn
Works as intended

Second /execute store result and /execute store success only work with integers, providing a string to it will only return its length
Works as intended

Documentation has been updated and is available on https://modrinth.com/mod/data-command-plus