OP: You know the best as to which joints and movements cause pain, so you need to find a position that is comfortable enough for you.
As others have already pointed out, a flat handlebar will be more limiting than a variety of others, which allow you to change your hand position after 10 min or whatever time span works for you.
Consulting a bike fitter is an excellent idea but if budgetary constraints prevent you from it, go a locally owned bike shop, where they sell new as well as used bicycles. I am fairly sure, if you get there during a time when they are less busy (avoid weekends), they will be happy to let you try various bikes in your size range and may even be willing to make adjustments to suit your needs.
If they are too busy the first time you visit them, ask them when would be a good time/day, set an appointment around a little before lunch time, and show up with a hot pizza. I’m sure they will help. Besides the food bribe, most bicycle mechanics are really helpful people.
