Hello I have found a very good tutorial: Access point as WiFi router/repeater, optional with bridge and I've managed to get wifi repeater working (w/o bridge)
My question is, how could I run my own DNS server like dnsmasq and send it over to clients on the network for adblocking, etc?
It should not be a problem to just install and configure dnsmasq on the Raspberry Pi in addition to the access point.
Then you have to give its ip address to the devices that are connected to the access point. In the example of the setup you have used the Raspberry Pi (with the access point and the dnsmasq you have installed) has the static ip address 192.168.4.1. Now tell the DHCP server to give this ip address as DNS server to the connected devices. In /etc/systemd/network/12-ap0.network just modify the line:
DNS=84.200.69.80 1.1.1.1
# to
DNS=192.168.4.1
Now the devices will use your DNS server.
