> value in € = 345 # valid with libconfini but invalid in TOML
This is degenerate.
You want a key-value format easy to pick up, and easy to edit by hand - otherwise you'd rather just use something like SQlite - in particular if you "need" the other insanity that is sections in INI files.
Why the heck allow spaces and UTF8 in key names? To satisfy someone's libido?