This is what I have been using for years : How to allow VLANS - #2 by nero355
And it looks like it's going to work fine for Pi-Hole v6 too in the future
I am hoping to test it tonight after solving some webGUI access issues...
/EDIT :
Turns out I did not need the old config file anymore :
Pi-Hole v6 binds to all my VLAN Interfaces by default : SWEET!!! :
However if you want the edns-packet-max=1232 line to /etc/dnsmasq.d/ in a seperate file then you need to do the following in /etc/pihole/pihole.toml =>
In the same /etc/pihole/pihole.toml you can bind the webGUI to just 1 Interface/IP address so it's not reachable from all the (VLAN) Interfaces if you want