My Ajungilak Kompact 3 season is rated to -5C and was just about perfect for N Europe late summer/fall. Its a bit too warm for those clammy S Europe summer nights.
The big problem for cycle tourists is where you transit through many climate zones, from lowland summer to high altitude and spring/fall. One bag just isnt flexible enough.
I think some kind of modular bag system is needed but nothing too complex or zippy. Maybe a summer synthetic bag with one or two down tops that can tie on. Insulation underneath is not so useful and you should have a good matress.
My matress system includes a 3/4 inflatable and a 1/3 length foam mat. I can use the foam to sit outside and if/when the inflatable develops a leak, I still have a fall-back.