First of all, I'd suggest you use Hockey instead of TestFlight. Although I'm sure the product will change now that Apple has acquired them, TestFlight is currently inferior to Hockey in so many ways, I could write an entire post on just that!

Ignore "cool early adopter tech people and friends and family" and decide which of the user segments of potentially real early adopters are most likely your customers. This is best accomplished by actually segmenting each group (into a separate email list) and surveying each to understand their current perceived degree of pain for the problem you're solving and also trying to assess their degree of motivation to experiment with your approach.

The product should be refined enough to fully function before testing with real customers. This is a point I can't stress enough. Customers act the same regardless of what stage of development you are in. This is to decide they will decide quickly (within minutes) as to whether they think that what you have on offer is worth their time. If you fail to impress the first time, you will have a difficult time getting them to reopen the app, regardless of what the build notes have on offer.

So if you're still a ways away from having a true MVP, you should count on early adopter tech people to give you generalized product feedback. Their feedback can't be relied upon to validate actual product/market fit but they can help you shape the MVP to get the core UX right and help squash bugs, etc.

Happy to talk through this in more detail in a call.