FWIW, I found full suspension more uncomfortable at first; too much movement. I can't remember which vertebrae you had fused, but mine were super low (L5-S1). I did (and still do) better on my commuter which is more upright than my road bike and has 32c tires. It's just so very heavy!

What hurt me the most was trying to push big gears. So using your mtb with slicks would probably be better than trying to ride a road bike with standard gearing. You could always get a road bike built with lower gears, but since you already have the mtb it makes sense to me to use that.

Good luck and take it easy.