Is there an option to make the triplet bracket curved (i.e. look more like a slur) instead of the boxy shape it is by default?
Not really, but I really do wish this was implemented…
No, this is not currently possible, and I confess it is not something that we consider a high priority. I guess you are trying to reproduce a specific older edition exactly? If your edition is intended to be a performing edition rather than a facsimile reproduction, I would urge you to consider using the current common practice notation for tuplet brackets.