I made some "confiture d'orange" style marlalade earlier this year, without peel, from sweet oranges. It's very nice, of its style, and not like Seville marmalade.
A few observations:
You don't need to add pectin to citrus jams and marmalades, pectin is made from citrus peels. Red fruits will give a soft set unless you add a bit of apple, minced orange, lemon or similar.
Commercial pectin should be avoided, it makes the jam far too stiff. You want a soft set. People add too much of the stuff and it's awful.
You generally don't need to add water that you subsequently have to boil off. My orange jam didn't have any, I just diced it up and brought it to the boil, at which point the juice releases. You can add your sugar at this point.
Sugar is essential, you need about 1:1 against your fresh fruit weight (neglecting the peel weight). Significantly less than this will not keep. However by all means experiment. The bitterness comes from the citrus peel content and not from the absence of sugar, bitter marmalade is still >50% sugar.
The set is a moveable feast, test it on any cool plate. Don't overthink it. It will also go quite quickly as you approach the set point, so watch it. It may also burn on the pan, be careful. Also be careful re safety, this stuff is at 130-140 deg C, it sticks like sh*t to a blanket and it will give you a proper burn. Treat it accordingly.