If you’re currently only using WaniKani, I’d say supplementing it with something is definitely a good idea; WaniKani is very upfront about being a kanji and vocabulary learning app, and doesn’t teach grammar in any tangible way.
I personally quite like LingoDeer, which teaches grammar in a nice and structured way. My main complaint about it is that it doesn’t teach much vocabulary, but you already have WaniKani for that.
As for anxiety … in my experience, that’s something that never really goes away; the more Japanese I learn, the more conscious I become of my own inadequacies.
The closest thing I’ve found to a remedy is going to Japan, so that I have no choice but to carry on anyway.
“Is a frappe 一つ or 一杯? What’s the pitch accent when 一杯 is used to mean ‘one cup’ as opposed to ‘a whole lotta’? Do I say ください or おねがいします?
Well, I’d better just say something, because the person in front of me is just about done.”