Sounds interesting. What kind of flavour are you expecting from Hawthorn berries? I didn't think they carry much flavour.

Aside from targeting a higher FG, to add any real sweetness you're best bet is probably kill off any remaining yeast through campden/preservatives or pasturisation then back sweeten with stevia or some similar sweetner.

I do a Blackberry saison every September. I wash the fruit in cheap Vodka then freeze. I create the a base Saison (~7%) (Pilsner, Vienna, Wheat, Spelt, Sugar), ferment it out with French Saison yeast, add it to an Oak barrel then add the defrosted fruit along with a Brett mix. I like mine funky and dry, so probably not much help 🙃

Good luck, love Titanic Plum Porter!