

It depends a lot how you use both words. For me:
- colloquialism - anything you wouldn’t use in a higher register of the language (e.g. when speaking with a judge, or when writing to a formal-ish audience)
- slang - colloquialisms allowed only in specific settings and associated primarily with a subculture
So one is a subset of the other. Some colloquialisms are not slang (e.g. EN gonna, tryna, wanna), but slang is always colloquialism (e.g. big dick energy, skibidi, bussy).
Note: that’s how I use both words. But not even linguists agree on their definitions, so don’t see yours as incorrect.
[unfunny guy analysing the whole thing]
The case with “with all due respect” is a lot like Japanese 貴様 kisama: misplaced respect is interpreted as ironic and sarcastic, thus rude.
Because, like, there are a few cases where “with all due respect” is genuine, but they all involve someone in a hierarchically inferior position contradicting a superior; student/teacher, underlying/boss, etc. In those situations the expression is there to convey “I acknowledge my position, and I’m not questioning it, but I need to say my honest views”.
On the internet, though? Hierarchy? Mpfffffft. [/unfunny guy]
inb4 I got the OP, on “all due respect” + no respect = “this is how much respect you deserve: ZERO!”. I’m just nerding out with language.