A second camera adds capacity only if a second trained person is holding it. One shooter can't be in two houses at once, so if he's the only operator, the hardware sits in a bag most days and functions as a spare.
Capacity for one shooter looks roughly like this. A 3D capture on a 1,600 square foot house runs 45 to 75 minutes of scanning on top of the photo work, because the camera has to be repositioned every few feet. Call it two hours on site for photos plus 3D, then processing and editing, then drive time. Three full listings in a day is a solid day. Four happens when the appointments are close together and nothing goes wrong. Five is not a repeatable week.
So test his 15 listings a week against that. Fifteen across five days is three a day every day, which is a shooter running at close to full capacity with no room for a sick day. Ask him what share of those 15 buy the 3D add-on, because the $250 line only shows up on some of them, and ask whether the 15 is a good week or an average one.
The thing that drives all of it is listing volume in his metro. This work is downstream of houses coming to market, and when new listings thin out, shoot counts follow within weeks, not years. A repayment schedule built on a busy stretch can meet a slow one.
Separately, structuring a repayment out of revenue is a legal document with tax and possibly securities implications, and that's a conversation for an attorney rather than a forum.