I'm a tractor guy too. A rider that checks all the boxes is a Deere X series w/ their 48" Accell deck. You can get this in an all wheel steer. Know that the rear bagger will be a powerflo system w/ a powered price tag. These are real nice tractors and decks though. Kawasaki engine. Kubota makes a nice 48" unit, will use a Powerflo type bagger system.
If you wish something more inline w/ what you have- just larger: A Deere S240 w/ 48" Kawasaki engine. You may be able to get a Husqvarna w/ a Kawasaki engine in a 48". That's what we have at home. Husqvarna's like to hold onto the grass under the deck- but are a well made deck. The tractor itself is similar in quality of your 120. Deere's E series have a good 48" deck which does not use a Powerflo system. This will have a twin Briggs engine though. I'm OK with recommending a Briggs twin, not so much their singles. I get though once a guy has been soured, you don't want to be bit again. Simplicity is made by Briggs. MTD / CUB /Troy are all cousins. Their 50" deck is pretty decent and would be similar in build quality to what you have.