EDIT: I think it's important to note that I am using Debian Buster with the latest updates to date.
I have successfully got PiHole working and have used it's DHCP function also working. It's great!
I also wanted to see if I could get my Pi working as wireless access point. I have been using this guide: https://thepi.io/how-to-use-your-raspberry-pi-as-a-wireless-access-point/
Pretty easy to follow but when I'm done, my PiHole stops working (See this Q: Why is the FTL service offline and the DNS service not running?)
So, I believe it's because of the 2x DHCP servers?? Even though I made the range 10.0.0.1-199 for the PiHole and 10.0.0.201-250 for the WiFi access point (10.0.0.200 is the wlan0 address).
My ranges are good and everything else seems OK but as soon as I try to set up the WiFi access point, everything breaks.
Got me thinking, can I even have this? Is it a hardware limitation? Does PiHole use dnsmasq and when I reinstall it for the WiFi access point, it just ruins everything? Is there a way I can have PiHole (to block ads through DNS), PiHole DHCP and a WiFi access point working at the same time?