I wouldn't do it with your hip replacement history. Get yourself some Five Ten Freeriders and some mountain bike pedals with pins and your feet will stick to the pedals way more than you might think. I use that combo on road and off--I ride a gravel bike primarily and tackle some pretty gnarly stuff fit for a mtb, and my feet never leave the pedals or shift unintentionally. I'm 58, and while I don't have a hip replacement, a falling over injury could really suck! I'm a musician and a broken hand/wrist would put me outta work. Granted, I've hit the dirt a few times trying something silly on the trail, but that's...different, somehow
Speaking of cool flat bar road/gravel bikes, I was just looking at this Niner--looks pretty darn nice:
https://ninerbikes.com/products/rlt-9-apex
I'm hooked on drops these days but I'd look at that rig if I was considering flat bar.