I'm not a farmer, but I live in the midwest, so does that count for anything? I'm guessing farmers aren't looking for 3d point clouds of their fields. A nice stitched map might be cool, but probably won't tell them a huge amount that they wouldn't already know. Most of the core products from places like drone deploy and pix4d are super cool, but maybe not directly beneficial to the ag industry.
From what I've seen of sentera, they are pushing in the direction of fly high, fly fast, get wide area coverage (with not much image overlap), and then they use what they call "direct georeferencing" to place the imagery. So if they know the location and orientation of the camera at the time a picture is taken and the approximate elevation of the ground, they can do the math and plunk down all the pictures right about where they should go. It doesn't produce a seamless perfect stitch, but should put everything in it's proper place within a meter or three. Combine that with true NDVI (or true NDRE) and you can get a pretty good sense of crop health over a wide area. You do actually need near IR or red edge reflectance to get a true measure of crop health. False NDVI is interesting and looks cool, but isn't nearly as accurate and showing true issues. Sentera also has developed a fixed wing solution which flies 3x as long as a quad copter, probably 2x as fast, and covers just that much more field area in a flight.
I'm not in the ag industry, so don't put too much stock in what I say, but if I was going to think about advice I'd give I would make the following comments.
1. Think about this from a business perspective first. Think about all the boring, difficult, time consuming, stressful parts of setting up and running a business first. You are going to spend more time doing those things than being out flying. What specifically you are doing for the business might even be a secondary concern if you don't get a good handle on all the business stuff first. (All that depends on how serious you hope to be ... is this going to be a spare time gig for a bit of extra spending money, or are you trying to go all in and support yourself 100% and grow from there?) You need to provide something that directly saves a farmer money, or is something they think they absolutely need.
2. You are probably going to be competing with crop consultants that have a ton of local and specific knowledge of crops, pests, fungus, irrigation ... they have experience to recognize all those possible issues for crop growing along with knowing good solutions. Your drone flights will probably get paid for from the same money the farmers would have used on an traditional crop consultant that would walk the fields. A farmer is also running a business, so any time they pay to bring in a consultant or outside help, that costs money, and if the information doesn't directly lead to costs savings at the end of the season, they probably aren't going to want to throw away money on things just because they are cool.
But I think there is an argument to be made for drones in agriculture for sure. Sentera and many others have built thriving businesses around that. You just need to show that the information and maps that a drone provides will save a farmer money. If they can spot-spray pesticides for instance, that costs them less, they get similar yields in the end, and bonus there are less pesticides going into the environment and our food supply. Drones are an important part of the larger precision agriculture revolution.
Farmers are making financial tradeoffs all season long ... if they see an issue in their fields, they are weighing the costs of treating it versus the potential lost yield. In some cases letting things go is the wiser financial decision (i.e. take an estimated 20% yield loss versus paying $x per acre to treat the issue.)
3. Sentera might be a good resource for a startup drone service business. They want to sell drones, cameras and analytical software, so I suspect they would be very helpful in showing prospective customers the best ways to use their equipment and how these tools can pay for themselves.
Full disclosure, I know some of the people that work at Sentera because it is a local [to me] company. I really like their company, I think they are doing some great things, and I pull for them every chance I get. (There are lots of other options out there too .... every company has strengths and weaknesses and sentera is no different in that respect.) I ended up taking a university job after treading water for a few years as an entrepreneur, so don't take business advice from me. But if you are just starting out, maybe think smallish, and part time (don't start out way in debt.) Build up experience, build up your fleet. Try to come up with a rate that is acceptable to farmers, but enables you to build your business. Maybe see if you can start out working for someone else just to get a year or three experience in the field.
Again, I don't know anything, I'm just talking here ... use what makes sense for you, just skip what doesn't. Go ahead and correct where I'm wrong, but it's too late for me anyway, save yourself.

Best wishes! It's super exciting to just be starting out, I'm sure you have some great adventures ahead of you!
Curt.