I have tried all of the major apps, and I use the exact same combo (UpNote, Things 3, BusyCal).
One special note is that I have been transitioning away from the calendar and moved towards putting all of my appointments in Things. I'm finding more and more than appointments are just tasks, so really it's just UpNote & Things 3.
I use Notion for work because of UpNote's 20MB requirement wouldn't cut it for me there.
Special Notes:
1. Can't wait for Things 4. Hope it's not subscription.
2. UpNote is amazing. Can't believe it doesn't get more love! With that said, the 20MB file size limit is bothersome
3. BusyCal is also great, both for Mac & iOS.