Sounds like your choice may ultimately be a limited one determined by availability for 5+ persons and proximity to a rail station...

This place in Titisee is fairly close in and available at the end of August:
https://www.vacation-apartments.com/120440/

Any hints on travel to Paris from Black Forest.
Any worthy stopovers or Direct connection from Offenburg?>

Well I'd head south to Basel and catch TGVs from there to Paris- Nancy is one really neat town and Reims of course with its cathedral and Champagne House tours. www.capitaintrain.com for easy bookings Basel to Paris - under regional tickets I suppose Freiburg to Basel?

<<Any hints on travel to Paris from Black Forest.
Any worthy stopovers or Direct connection from Offenburg?>>

Well I'd certainly stop over in Strasbourg. It's just a short hop from Offenburg. From there you can take the TGV to Paris.