Agree with Ron H . . .
Contrary to what you may think, a wide/soft/squishy saddle is NOT the answer to comfort.
More riding will toughen your butt muscles.
On drop bar bike you even out body mass/weight between the lower bar postions, pedals and saddle.
On Mt. bike, because you sit upright with flat bars, most of weight will be on the saddle/butt.
Getting some bar extenders on your mt. bike will allow you to lean forward more, give you more hand positions so all weight not always be on your buns.
Perhaps it would be nice if you lost some of that 230 lbs . . .
BTW I turn 80 this year and have ridden drop bar bikes for over 300,000 miles.
Stick with it!