If there are no properties like yours (size, amenities, etc.) then it can indeed be difficult to comp it. To some extent you can look at $/sf.
I agree with the poster who said it’s not new construction - it may be newer than others in the area but it’s been lived in and is no longer new.
The location matters for sure, but all those things you’re saying add value may not actually add as much value as you believe.
Likewise, rents don’t matter when it comes to valuation on a residential (maybe to an investor but not to an appraiser).
Like others said, Zestimate can be wildly inaccurate, especially on a unique property or one that is especially dilapidated or upgraded.
Instead of asking agents to comp it, pay a broker for a Broker Price Opinion (BPO). It costs less than an appraisal (which I would spring for if you have the funds and honest intent to price it right) and is paid by a flat fee, not a commission. Although I would say if agents are running CMAs for you, it’s in their interest to get it as high as they reasonably can since their commission is based on that...so it seems they wouldn’t lowball it on purpose. But if they feel you’re just shopping around for agents to do free work, they might not put much effort into it.
I personally wouldn’t expand comps to a 3mi radius but perhaps that’s what is necessary if the home is unique.
Sounds like some of these agents youre dealing with are legit flaky but I think part of the problem is your expectations.