Buy a new (used) bike or modify what I’ve got?

I’m 5ft 11 and 3/4in with a 32in inseam. I know - I have pretty short legs/long torso for my height.

I bought a used Haro Beasley DLX 27.5 XL used on marketplace. I rode it around for 10-15 min and it seemed solid to me.

Then I did an hour long ride and it bugged my low back. Adjusted the saddle as far forward as I could, then went on a 1.5 hr ride a few days later. I felt like I was reaching for the handlebars and wound up with some discomfort in my shoulders/elbows/neck.

My goal is to do some bikepacking this summer, so I want the bike to be comfortable for longer distances. I’m looking to do gravel and off-road stuff this summer, but I currently ride mostly on paved bike paths.

I’m currently caught between the option to sell the bike I have and get a size Large of either a Giant Roam 2 or a Cannondale Quick CX3, or buying an adjustable or shorter stem and the Jones Loop H Bar installed on my Beasley. Thoughts?