It sounds like you'll be better off with drop bars and either more aggressive 1x gearing or 2x gearing. You can do fast starts/sprints on a flat bar bike (I've ridden my wife's flat bar road bike and my 29er is set up for pavement and packed gravel), but I think they feel better/faster on drop bars due to the wrist angle. Given your riding, you'll want beefier tires, too, especially if you're losing the suspension fork. Maybe 38+? I have 32mm on my cross bike and 44 on my 29er.
Go with a gravel bike or cyclocross bike with disc brakes, preferably hydraulic.
Any idea how fast you're going when you run out of gears? That will help figure out appropriate gearing. Also, if you get clipless pedals and bike shoes, you'll be able to spin the cranks faster, comfortably, which will improve your top end.