I want this to return as a string

[cursive wedge]

"I want this to return as a string"
return {_progressBar}
The function returns a string, exactly as you said it should.

.
abiguity aside, that's not how you add things to a string. the add effect is for math and adding elements to a list. this is neither, unless you want to make a list of strings and then join them at the end.
you want something like set {_string} to "%{_string}%EXTRA", which will append stuff

[cloud obsidian]

is there any way to remove the 1 pixel spaces between each unicode character?

[cursive wedge]

bold them?

[cloud obsidian]

nope

In another function i used text components with a custom font

  set font format of {_p1} to "mcc:negative_padding_nosplit"```

how would I incorporate that?

[alpine quarry]

did u use a negative space texture pack?

find the unicode character online that its replacing