What is considered long will change with the bike’s wheelbase (which depends on rider height and bike purpose, among other things).
Rear center to front center ratio (search RFR on here) has proven to be a useful metric for me when evaluating weight distribution. RFR in the 1.5-1.7 ish range could be a ‘long chainstay’ design IMHO…, which sounds well suited to your design goal.
good thread on the topic:Chainstay Length Discussion - #10 by Daniel_Y