I know I'm waaay late to the party, but since this thing seems to be treading water lately, I took the survey anyway. Here's a little more explanation:
1. USB port. Whatever is smallest, lightest and cheapest. I have and plan to keep for quite awhile digital cameras that use Micro, so my cables aren't going anywhere. I don't think I have any C cables yet. I don't think talking with the device will include enough data to benefit from the speed of USB 3.1.
Here's my interest:
I'm an engineer and vehicular performance nut, always wanting to do more with less. I am into efficiency. In rockets, that means getting the max altitude out of small ones. Like staging 18 and 24mm MD rockets, including composite and reload motors. Looking at apogees anywhere from 2000 to 5000+ ft, depending on configuration, and maybe higher if I get silly. I want to be able to document what the rocket actually did, and get it back. So logging data and GPS to find the rocket and the data repository. Don't expect to ever fly a camera, so I'd rather have smaller/lighter/cheaper onboard memory than a micro-SD slot.
I can launch in a park across the street from my house, but only a few hundred feet without it ending up in a live oak or someone's yard. Might be useful for shaking down a recovery system or something, but boring. Minimalist testing only. The places where I will do most of my launches are dry lakes in the Mojave. I don't mind wandering around in the desert, especially if I know where I'm going.
If I need dual deploy, it will probably be Quark-based. I can do Eggfinder Mini. If there's a smaller, lighter, cheaper option with a cheaper, easier to use ground interface to tell me where to come get it, that would be awesome.
I'm also interested in the accelerometer function. One thing I'd like to do is make a machbuster without busting the bank. I have a lot of sims that get to Mach 1.0x on reasonable 24 or 29mm composite motors, but baro- and GPS-based altimeter systems both have trouble accurately recording and reporting transonic and/or supersonic velocities, as I understand it. So if I build the rocket that sims at Mach 1.03 to 1.07, there isn't any way with just baro and GPS to know that it actually flew that fast.
I've been looking at the Raven with its accelerometer, but it's kinda expensive for just that, and pairing it with an EF Mini adds a lot of size, weight and complexity. Plus, at that point, you're at ~$235 of electronics in the rocket, not even counting batteries, wiring, etc. There's also building models that sim to Mach 1.2x-1.3x, and assuming that if the parts of the trajectory I can measure reliably reasonably match the sim, that it at least got close enough to be confident it was >Mach 1. But it would be a lot cleaner and more elegant to just have one unit that would track speed accurately into transonic and to Mach 1.0x with an accelerometer, and also tell me where to come pick it up. Having it smaller and lighter than either of the above units would be super gravy. In general, "optimum mass" for apogee doesn't apply if top speed is the goal. Lighter is better.
I suspect if one was to fly the no-GPS basic version of your board on a Mach 1.0x flight, it would still be an issue to find the rocket, even if the accelerometer provided an accurate speed recording. Adding your GPS daughterboard would be better-cheaper-lighter-easier-compacter than using the base version with an EF Mini, even for someone who already has an EF Mini.
Another example: An 18mm MD two-stage with efficient fins sims to >3100 ft. Only Mach 0.63, so no worries on the speed recording. I have an Altus Metrum MicroPeak that fits easily in an 18mm nose cone, but it's baro-based, so unreliable for transonic speeds, and it doesn't tell me where to come find it. An Eggfinder Mini with just a piece of heat shrink over the delicate parts to protect them doesn't fit in a BT-20 and definitely doesn't fit in a BT-20 nose cone. Taking off the heat shrink and stuffing it in the tube requires lengthening the rocket considerably, which will hammer the apogee performance. Your hardware, as you've described it, would be perfect!
Yet another example: I kinda wanna send a Goblin as high as I can. But due to the short body tube, space gets really limited when you stuff a long motor in it. Also, the way to max apogee is a slow-burn motor, but the optimum weight with slow burn is considerably lighter than with a fast burn. Your system, again, seems like a great answer.
If you're going to create designs for printing nose cones, it would be cool if they were drag-optimized. One for subsonic (elliptical - the one semi-definitive statement I've seen about optimum fineness ratio references the 1973 MIT authors) and one for trans/supersonic (probably parabolic or hybrid), as the best designs are different in each realm. Kinda frustrating that most commercially available nose cones are optimized for what low-information muggles perceive to be aerodynamic, rather than actually being optimized for minimum drag.