How do you know when to pronounce the s?

Bonjour Torbjørn !

In the case of "de plus en plus", here's the pronunciation rules:
- the first "s" is pronounced [z] as a liaison is made with "en"
- as for the second "s", it depends:
-> if it's followed by an adjective/adverb ("de plus en plus gentil" / "de plus en plus vite"), then the second "s" is MUTE.
-> if the expression is used on its own ("Il fume de plus en plus."), then the second "s" is pronounced [s].

I hope that's helpful!
À bientôt !

But in the examples the second s is pronounced before the adverb lentement. Also, in another example, the s is pronounced before the preposition, avec----is there a rule for the second s followed by prepositions? How strict are these rules? I just feel like winging it. Thanks for your advice. I love KWIZiq!