In a search for a configuration for your
diagram above (without reading the first
few posts again). I did a web search :
....
pfconf pfctl 192.168 "gateway box"
....
On the first page, this link:
www.drones.com/openbsd.html
LOTS...LOTS of information (some maybe
outdated. ) You may wish to compare its
setup with what you have done so far.
....
The reason initially for the search was to find a
very large and commented pf.conf containing
192.168... I do not know whether the search
found one larger than that in the page linked
above.