BBQ fat on glass is not easy to deal with - think oven glass - WFP by itself won't touch it, and even doing it trad is a sod

Use something such Oilflow or TFR, but diluted and not neat, put in a hand sprayer and spray on & leave to soak for a couple of minutes while you do the windows around it.
Use something like a rough kitchen pad...what are they called!...you buy them in packs from your local Tesco and so on...anyway, not the really abrasive one, just one that gives you some extra scrubbing power.
I use these on mirrors I have to clean on a local Greggs cake shop I do, and been doing so every week for years too and not scratched the glass (before anyone wonders!)
once it has soaked for a couple of minutes, give it another spray and quickly scrub the window, leaded or not won't particularly matter.
Hold your WFP brush off the window and just rinse first (just to get rid of any excess detergent) then wash as normal.
You'll have to wash & scrub a little longer than normal of course because there will still be detergent getting on the bristles.
Should only add a few extra minutes to the job in total.
I've had these windows in the past where I have scrubbed and scrubbed, and even used a razor knife on them - and that's on a purely squeegee window - and there are still spots from the BBQ fat left behind
So if they are cooking out there all the time, don't expect to get it perfect.
Ian