Clydesdales/Athenas (200+ lb / 91+ kg) - Considering New Bike...Gearing? Flat Bars?

I've signed up for Ragbrai, thinking I may need to look at a new bike. I'm about 220 lbs, and 52 y/o.
I have Schwinn Frontier mt bike, about 15 years old, and a Bianchi Strada LX road bike, probably 20 years or so old.
I feel more comfortable on my mt bike, like the upright position, easier on my back. Road bike rolls nicely, seems comfortable, but I seem to have more pains after a long ride on the road bike.
Saturday, I did a 65 mile ride on the mt bike. Fairly wiped out afterwards.
Just wondering if most heavier bikers find a more upright riding position more comfortable, and if they need lower geared bikes.
I've been looking at the Giant fcr series. Not done a test ride yet, but I think I'd like the flat bars on a road frame. Living in NW Arkansas, I need low gearing. I've seen cassettes for 8 or 9 speeds that will go to 11-34.
Just wondering if heavier cyclist go for flat bars and low gearing.

I am heavier than you at 6'3", but I ride my road bike more than my others combined. If I was doing RAGBRAI, I would only take my road bike. Since it is coming to my home town and my band is playing, I decided I wouldn't ride at all. I am also having to work at the YCMA as we are having a million or so campers that day also.

Like flip18436572, I would also recommend a road bike for something like RAGBRAI. That said, my road bike is pretty comfortable. If you're experiencing discomfort, it might be time to pay for a professional bike fit. Small changes in stem length, stem angle, handlebar width, cleat placement, saddle height, saddle placement, etc. can make a big difference when you're going to be spending a lot of time in the saddle!

My road bike has a compact double (50/34) chainring setup combined with an 11-28 cassette. This setup climbs very well for me, though obviously the 34-28 gear combo on my bike is a far cry from the 22-34 combo available on most mountain bikes.

I would get a plush road bike for a ride like that. All major brands sell them (Specialized Sequoia, Giant OCR3, Jamis Ventura, etc.).

You don't have to get a brand new bike, though. You can get an older bike on Craigslist or something and just set it up with a taller stem for a more upright position. If you are new to road biking, I would also suggest a triple crank. You can switch it out later if you want, but it will give you more gearing options for starting out.

Good luck.