I do a fair bit of my writing on an iPad Air with either Ulysses or Scrivener. I agree that the iPad is generally best as a device without a keyboard, and I don't like the added thickness of a keyboard case. The best compromise for me has been the iPad with a smart cover and a magic keyboard in a slim, lightweight sleeve. It's easy to throw the keyboard in my bag when I want it, but I don't feel like I need to constantly remove a keyboard case for my typical iPad uses (reading, research, internet, games, etc.).
TLDR: This may be the compromise you're looking for:
https://www.studioneat.com/products/canopy