I may retract my opinion that w123's are not the best cars ever
25 mpg at 70 - 75 is awesome for that car.
Firm shifting, if you can live with it is easier on the transmission I was told by a tranny guy. But if you want to soften it, it is a modulator valve on the right side of the transmission where the vacuum line connects. If it only does it when you are giving it a lot of throttle , I don't think it is a vacuum leak problem because when you give it throttle it takes the vacuum away from the transmission. The little modulator on the side of tranny just has a cap that pops off and you pull out the metal t thing and turn it a couple notches counter clockwise to soften the shifts.
722.3 and 722.416 series transmission adjustments
It is super easy to do with the car on ramps and you only need a flat head screwdriver for getting vac line and cap off. Maybe some pliers.