Why is my bread dough sticky?
I've never tried making a bread dough with that much sugar in it. You're up to ~80g sugar, compared to 140g flour (a "baker's ratio" of 57%).

It's possible that much sugar interferes with gluten development. Since you're already adding butter later (at around 4:07 in the video), the first thing I'd try is to mix the extra sugar and butter together, and add them both there. That's probably being done after some of the kneading because butter interferes with gluten development. So if the sugar does too, that may help.

Also, sugar definitely makes yeast more active (they're eating the sugar). So you probably need to let it rise for less time. Go by how much it's expanded (risen), not by the time.

Second thing I'd try would be a tad bit more flour. Knead in a little more and see if the stickyness returns to manageable levels.

Third thing would be to use some artificial sweetener. Those are substantially sweeter (per gram) than sugar, but of course have aftertastes... The combination of some artificial sweetener and still plenty of sugar will mostly mask the aftertaste.

Fourth, similarly, different natural sugars have a different sweetness level (per gram). Fructose, for example, tastes sweeter than sucrose (table sugar). A mix supposedly tastes sweeter still.

Finally... consider just serving it with syrup! Or icing or frosting it after baking.