He’s more similar to Sterling than Sané IMO, and here is why:
Your simplification of “on the ball winger” vs “off the ball winger” is, while accurate to a degree, just too oversimplified.
Doku is somewhat a mixture of Sterling and Sané, but on the ball his style is far more similar to Sterling. Doku gives the ball away rarely like Sterling, and unlike Sané.
Doku’s shooting technique is similar to Sterling’s, albeit a little bit cleaner.
Doku is a smart player who takes up very good positions (see his goal from the weekend) and has a high xG, which is more similar to Sterling than Sané. Doku’s pace is probably slightly quicker than Sterling but slightly slower than Sané. Doku is also a very effective dribbler (just like both Sterling and Sané).
Additionally, he’s right footed and a similar height to Sterling (not that it really affects his game too much).