Well, to wrap this up, I just purchased a Kubota Z724. I'm having a bit of immediate buyer's remorse only because while I'm normally good to go for buying a thing one level above what I actually probably need, I think I've probably gone two levels here. However I was in more of a hurry for this purchase (since I'm losing $150/week while I dither), and after driving both a Z421 and a Z724, the Z724 felt more solid and responsive under my weight. Certainly it's more of a tank than the Z400 series, 8-gauge deck for example, instead of 10+ (it isn't exactly 10) for the Z421, upgraded transmission systems, and what-not on down the line.
Buying off season, and in the pandemic, means all the dealers are out of stock and I couldn't try much of what I wanted to try, e.g. the eXmark Radius line. Anyway, hopefully this is a solid thing with long term resale value if necessary. Time will tell. It certainly flies (time, and the mower!).
Thanks for the help all. Any thoughts on whether I've done good or bad, let me know. The mower was $8849 after promotions/rebates, ignoring tax & delivery. 4 year warranty.