You only need 1 brush for window cleaning

I use a Gardiner super lite extreme soft flocked hybrid. Nice and light and easy to use.
For 8-10 week jobs or first cleans. I use sill brush again soft and flocked.
Lead will come off regardless but a stiff brush will simple do more damage long term.
You can use less pressure with a stiff brush in leaded windows but why when a perfect brush for lead windows is on the market.
I have a selection of nearly 20 brushes on my van and each one has a purpose.