jumping in as someone that's baaaasically trilingual (at least, in terms of writing/bilingual (or multilingual if you count dialects that family members/in-laws speak) in terms of speaking, i actually think that "low-level" lit can be really useful for learning the foundations AND learning how to write in a way that is accessible/easy to understand. there's a time and place to be flowery or to be straightforward but poignant.
also hi fellow french speaker!
in canada, public schools tend to have at least 5 years minimum of french education. we were taught with kids shows and "simple" books like le petit prince (which btw is a 10/10 classic book, i'm aware my tastes are basic lol) or even reading classic books in french, that type of thing. but you seem like your understanding is already pretty good!
i'd recommend listening to music (i guess ~sexy~ music if your focus is on being better at writing smut?) and conversing with people fluent in the language! what sounds sexy in english? if it sounds sexy, it probably reads sexy.
on thesauruses: thesauruses might be useful to find similar words, but i think talking to a native speaker (or reading synonyms in a sentence) can find the nuances of certain synonyms. take happiness for example: thesaurus.com states some synonyms for happiness like "elation" and "contentment". but i'd say someone pretty familiar with english would know that they describe different types of happiness.
another niche french thing: maybe you could also practice writing smut in your native language? or reading smut in both languages and seeing the similarities? i think it might be helpful to see if metaphors/euphemisms can be transferable and see if that expands your range in storytelling. for example: la petite mort aka the little death (ik, ik, an old term), but sentences/ideas can be formed from that like "dying from pleasure" to describe an orgasmic state. stuff like that. the possibilities are endless.
tl;dr - write more, read more, enjoy more art, and get creative!