[metro424]

Both motors you listed are residential grade- might be worth jumping up to the next level.

Those Scag machines are really nice- maybe a bit overpriced.
Not a huge fan of the Kohler motor, but I know they sell thousands of them.

For the mid $5K price range, I'd start looking into machines with at least the Kawasaki FS series of motors, which is a step up from the FR series in the Gravely ZT HD.

Not sure if this is an option where you live, but look into the Ferris line.
When I did extensive research on my machine, nothing could touch them, as far as value and feature.

I got the Ferris IS600Z, which has suspension, Kawasaki FS600 motor, and ZT3400 hydros, 48" deck for $5,500 new.
Drives at 10mph which is scooting and has big 'ol 5.5gallon tanks.

They've added an IS400Z for $4,700 that has the FR651 and the same hydros as your Gravely ZT HD.
https://www.ferrismowers.com/na/en_us/product-catalog/zero-turn-mowers/400s-zero-turn-mower.html

Anyways, I'm a huge fan. Build quality is great and suspension is well worth it after a long day.

Edit-
Here's another option that is really good value.
Cub Cadet with EFI- fuel injection
$5,800 for a commercial mower.
https://www.cubcadet.com/en_US/commercial-zero-turn-mowers/pro-z-148-l-efi/53TWEHRF050.html

Also look at the Husqvarna MZT52 (too big?) Around $6K.
https://www.husqvarna.com/us/products/zero-turn-mowers/m-zt-52/967177006/

Holy cow, this thing is a tank. Almost ALMOST bought this one, but just a bit bigger than my needs.
You could gobble up the grass. FS motor, ZT3400 hyrdos.

I have a husky mzt52 with kawi engine and would absolutely buy it again. I've put 200ish hours on mine with no issues what so ever. You're getting a light duty commercial mower at the same price as a high end residential mower. The FS series is supposedly a commercial engine and zt3400 hydros are fully serviceable and have a much higher weight rating so you can actually do light towing without exceeding specs. I read some complaints that the seat pan cracks and isn't covered under warranty but I've never had an issue. Test drive them if you can because if I ever replace mine I'll probably buy one with a suspension next time!