Just to make sure, do you pronounce plus with or without the ending /s/ in the phrase "plus de l'avant"?
My understanding is that "plus + NOUN" leads to the ending /s/ sound being pronounced, but "plus + ADVERB" results in no ending /s/ sound.
The word "avant" can be an adverb and a noun.
However, the indefinite article "de l'" suggests that "avant" is a noun, so I am guessing that "plus" is pronounced with the ending /s/ here.
Please let me know if this is correct or if I am mistaken.
To me, it depends on the meaning of the word rather than on grammar.
"plus" without the /s/ sound means 'anymore' (like "Il n'y a plus de chocolat", "There isn't chocolate anymore")
"plus" with the /s/ sound means 'more' (like "Demande-lui plus d'argent", "Ask them more money")
In your sentence, "plus" means 'more' I suppose? As in go further? If that's what you meant, then you should pronounce the /s/ sound at the end.