Regex replace all on token list which is used as argument for command/function/environment

Question

I am working on a package with a command which creates a new tikzpicture. I would like to pass a list of tikzpicture arguments to the command. For example, for my single argument command, I might pass scale=1.5,line width=20pt,ultra thick to it. Next, there is a replace all regex call. For this MWE, I have simplified is as replacing spaces with a ~ character.

The problem occurs when I try to use the token list which has been regex modified and put it into the optional argument list of the tikzpicture. For reasons I have yet to understand better, calling \tl_use:N in this setting creates an error where the entire token list is treated as a single pgf/tikz key. For example, scale=1.5,line width=20pt,ultra thick as the argument will generate this error:

! Package pgfkeys Error: I do not know the key 
'/tikz/scale=1.5,line~width=20pt,ultra~thick' and I am 
going to ignore it. Perhaps you misspelled it.

I am assuming this is a catcode problem? Anyway, I'm fairly new to latex3 and may be missing something or approaching this the wrong way. The following is a MWE showing the issue:

\documentclass{article}

\usepackage{expl3}%
\usepackage{l3regex}%
\usepackage{xstring}%
\usepackage{xparse}%
\usepackage{tikz}%

\DeclareDocumentCommand{\mycommand}{ m }
{%
    \tl_new:N \l_tikzvowel_fix_spaces_tl
    \tl_set:Nn \l_tikzvowel_fix_spaces_tl {#1}
    \regex_replace_all:nnN { \s } { \~ } \l_tikzvowel_fix_spaces_tl

    \begin{tikzpicture}[#2]% <--- this compiles fine
    %tikz stuff
    \end{tikzpicture}

    \begin{tikzpicture}[\tl_use:N \l_tikzvowel_fix_spaces_tl]% <--- this causes the error
    %tikz stuff
    \end{tikzpicture}
}

\begin{document}

    \mycommand{scale=1.5,line width=20pt,ultra thick}

\end{document}

Calling \tl_show_analysis:N \l_tikzvowel_fix_spaces_tl on the token list yields:

The token list \l_tikzvowel_fix_spaces_tl contains the tokens:
>  s (the letter s)
>  c (the letter c)
>  a (the letter a)
>  l (the letter l)
>  e (the letter e)
>  = (the character =)
>  1 (the character 1)
>  . (the character .)
>  5 (the character 5)
>  , (the character ,)
>  l (the letter l)
>  i (the letter i)
>  n (the letter n)
>  e (the letter e)
>    (blank space  )
>  w (the letter w)
>  i (the letter i)
>  d (the letter d)
>  t (the letter t)
>  h (the letter h)
>  = (the character =)
>  2 (the character 2)
>  0 (the character 0)
>  p (the letter p)
>  t (the letter t)
>  , (the character ,)
>  u (the letter u)
>  l (the letter l)
>  t (the letter t)
>  r (the letter r)
>  a (the letter a)
>    (blank space  )
>  t (the letter t)
>  h (the letter h)
>  i (the letter i)
>  c (the letter c)
>  k (the letter k).
<recently read> }

What am I doing wrong here? And what is the standard latex3 approach to this type of situation? Although this particular situation refers to tikzpicture environment options, I'm an interested in a solution that could apply to any command or environment's argument(s).

Answer 1

This is the 'usual' issue with saving tokens for a keyval argument: keyval methods work very hard not to expand their arguments, so your setting are not seen. You therefore need to force expansion, for example

\use:x { \exp_not:N \begin{tikzpicture}
           [\exp_not:V \l_tikzvowel_fix_spaces_tl] }

(\begin is not engine robust so we need to avoid premature expansion).