I like your way of thinking, jfb - you really try to find a solution within the given limitations and available feature sets of Pi-hole 
For now, I chose to follow your suggestion and enabled DHCP again, as it was helpful anyway when trying to assist with Pihole plus Guest Wifi on different IP range
I dimly recall I had my reasons when deciding to use Pi-hole as DNS-server only, but I can't recall why (lack of documentation on my behalf 
).
I'll keep playing around with it for just a bit more and let you know if this fits my setup.
Thanks for your help so far!