I don't want to sound offensive but at 280lbs bike comfort is the least of your problems. As a physician what I see is obese patients think the world needs to be adapted to their toxic lifestyle when the truth is that 99.999% of an obese patient's problems and sufferings or discomforts and inconveniences is because they eat too much (sugar) and thus weigh too much. Kudos on starting cycling but it's a very particular activity. Some of it is just that you need to get used to sitting on the saddle. Yes you can get a better saddle but in the end if bumps are uncomfortable on a mountain bike you need to get in the ready position IE when you hit obstacles or "features' you need to get off the seat and not be sitting in it when the trail gets bumpy.

And even if you buy a better saddle as you lose weight and maybe get new clothes the fit will change and you may need a different saddle or setup. Reemember 280lbs will not only shorten your life expectancy but it will cause you an undue amount of suffering and disease. At least physically speaking everything in your life will be made worse and more difficult than if you weighed under 200lbs. Fix that first, everything else will either just automatically fall into place. Even if just losing the excess weight doesn't make all if not most of your physical problems go away then whatever residual issues may bother you will be way, way, WAY easier to get adjusted.