Everyone is different. I asked this question in an email to members of my cycling club last year and received all sorts of answers. The common theme was "layers." I would also add that you will need a bike bag (handlebar bag, trunk rack, etc.) large enough to carry the layers if you decided to remove them or you want to pack them at ride start in case you want them later.
I keep a log for rides in cool weather. I note the weather conditions, time of day, what I was wearing and whether or not I was comfortable.
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles