For those who actually need something like this in production, there is Snek: <https://sneklang.org/> “Snek is a tiny embeddable language targeting processors with only a few kB of flash and ram.”
Or you just forbid mixing spaces and tabs in the same indentation sequence, the way most whitespace-sensitive languages seem to end up doing. Or you make a slightly more reasonable rule: spaces may follow tabs, but no tabs may follow a space. That's at least unambiguous.
As implementer of an interpreter, did you feel that whitespace for lexical scoping made the job of writing the lexer significantly more complex?
<space><space><tab><space>
is different than
<space><tab><space><space>
So you also have to track the actual sequence of counts of white space used for each level, rather than just a simple count.
in my view, both are the same, both `is` (or ===) an IndentationError raise
This is not Python, or even within three orders of magnitude of Python.