Opinions follow, your situation may be different:
Buying a mower deck for my CUT was a very dumb thing to do. Wresting it on and off isn’t a whole lot of fun. Installed it pretty much limits work to graded/flat surfaces, else the probability of damage is high. Puppy wasn’t inexpensive either. From a cost standpoint, the hefty price paid for it would have gone a long way towards buying a quality standalone mower. If we’re not talking hills and dale, a stand alone mower could have been cheaper.
Pulling/installing the deck isn’t rocket science, but it is heavy, awkward to move into and out of position, and is fiddly to adjust. Maybe all that's changed with the drive over option. Once up and running, the quality of the cut is v good, better than my X500.
As mentioned, tooling around a yard with obstructions (trees, well, septic, house etc.) in a long vehicle require more operator attention. Can’t go under things with the tractor due to its height. Always thinking about ground compaction when traversing near the leach field. Industrial tires leave marks under certain conditions. Which is why I mow with the X and put up with the not so great cut.
Mow 2 acres with the X, use the tractor for grading, skidding, brush/tree collection, hogging and everything else on the remaining 38. Used the MMM three times over 18 years, look at it daily hanging from the wall.
Bottom line is because of frequent off road uses for the tractor, having a MMM deck installed is a no go. And it’s hard enough to install/remove that getting a dedicated mower was worth the cost. To me.
You may be different.
I'm assuming the non lawn portion of the 3.5 acres is wooded and or requires some sort of ground engagement, thus the need for a tractor?