To be honest, the amount you can get away with regarding fictional languages is always going to depend on your reader. Most readers won’t notice inconsistencies or mistakes unless they’re really glaring (using the same word to mean two different things without some sort of explanation, the language being consisting entirely of English cognates, etc.) although a few will notice how the language doesn’t hold up to scrutiny unless you put in the same amount of effort Tolkien did- which you probably shouldn’t, because Tolkien created the Lord of the Rings and the Simarillion after creating his own language, in small part just to explore the sort of culture that would use that language- so I wouldn’t worry too much about disappointing the linguists in your audience unless you’re specifically trying not to. If the story is good, they should be willing to look past a language with flaws.
I would suggest creating as much as you think is necessary, and then a little more if you want to or you think it’ll be more immersive. Looking at Dovahzul from Skyrim as an example is a pretty good ideas, because it’s a great example of a conlang that looks and sounds alien and strange, despite being built from basic parts.
It’s ultimately up to you how you’ll handle it, but I wish you the best of luck, and would like to remind you that writing is supposed to be at least a little fun, so don’t stress yourself out too much.
