I tend to a lot more lurking these days around here (I am still around though lol). However, if you look through my post history you can see some of my experiences with Kava and coffee.
Unlike most here it seems, Kava (by itself) messes up my sleep patterns. I can’t seem to get into a good deep sleep when I have a shell a few hours before bed. I have followed this non scientifically with a smart watch. My REM sleep will increase about 30 or more minutes, but deep sleep will drop to below an hour on my typical 7.5 hours of rest. After a few nights like that, I begin to feel fatigued and brain fogged. So no way will I personally mix the two at evenings.
I keep my Kava time to days I can sleep in a bit. That being said, I still use about 1 tsp of cheap micronized (Kona from Amazon) in my morning coffee. It adds abit of a better kick and helps keeps my muscles from cramping. Kava Kandies are my go to when I have a flare up or want to add a little Krunk Necromancy to a good Friday night kava session.