Tires should be wide enough to provide a smooth ride. Depending on surface that could be 28mm to 45mm. Wider than necessary isn't really helpful IMO. Wide low pressure cheap tires are like riding pulling a plow. Even that can be okay if you gear way down and don't go very far.

Have you adjusted your speed expectations, to align with the new attitude? If not, this is a problem to solve. Miles per hour is not relevant.

If your butt hurts during a ride, get off the bike for a bit. It'll be fine.