Lingua Franca Nova
Lingua Franca Nova (abbreviated LFN, also known as Elefen) is a Romance-based conlang first created by American C. George Boeree in 1965, but not published until 1998. It features a highly simplified Romance grammar influenced by Romance creole languages. It takes its vocabulary from five source languages (French, Italian, Spanish, Portuguese, Catalan) and tries to find a form closer to the original Latin where possible, and the most neutral/simplified where it’s not.
I have seen some criticism of Lingua Franca Nova that it’s just too simple, to the point that it can be hard to decipher one’s own notes written in the language if reread some time later (like, there are too many possible meanings). The number of words that double as nouns and verbs, and the lack of distinction between subject and object pronoun forms, are reportedly two contributing factors to this.
On the other hand, LFN has attracted an active if small community around it and there’s even someone who’s written two original novels in the language.