Nope, no reason to spend the extra money for what you want to do.
I say this as someone who helped write the D3 service manual and taught the HOG service class for 10 years.
I never liked the Zenith 1st stage because of the cover design. That center purge is a pain in the butt with gloves and it traps dirt, salt, and other crud. It can also get pushed in and dislodged if you get someone in a panic who really smashes it in.
Once the Classic came out with that all-silicone cover, ALL of my Zeniths got switched to that cover, and the Zenith covers disappeared. I still may have a couple here somewhere I haven't gotten around to tossing away.
I dived it because I sold them as well.
I was an early HOG dealer as well until I retired in 2022.
After that I only sold the regs ala carte (1sts, 2nds, and hoses) and tried to steer people away from the Zenith. The reg is the same reg.
Same case, same internals. The Classic is just better IMO.
The D3 is a sidemount reg. It is used for doubles and singles, but the primary reason for the D3 is that the turret swivels 360 degrees with hoses on.
That is not a concern for singles or back-mount doubles.
The D1 is a workhorse and a proven design.
I had customers taking it to over 300 feet and thousands back in caves.
And don't get the octo version of the Classic. Get a D1 and two Classic seconds with rubber hoses. The braided are a gimmick. They take a set, are floaty, will start to fray at some point, and are not friendly to small hairs on your neck or neck seal if you run a long hose. The miniscule amount of weight you save is not worth the aggravation or extra cost.