Here's my rule for numbers.
100 target customers that I've done customer discovery interviews with.
10 customers (who've paid / given me money)
25 who are active beta testers.

I usually use that order of things as well. I like to get early adopters to give me money to validate their feedback, then 25 to give product feedback to test usage & retention, but in parallel I talk to as many potential customers as possible to learn.

Getting enterprise customers into early TestFlight builds is generally speaking not recommended because first impressions often become lasting ones. The reality of it is that even with good product/market fit, most people are too busy to play with build after build in the early days.

Assuming you have validated the core product thesis before building the app, I would try and recruit whomever you know to be great product people into TestFlight first. Building a great mobile product is exceptionally hard and I've benefited so significantly by having so many of my TestFlight users be great mobile developers, and product owners. There is a lot of very generalizable experience around what makes a great mobile app, and having people who can quickly spot bad UX will reduce your iteration time significantly.

For mobile, it's important that the polish is there before onboarding users whose favorable impression might lead to enterprise sales. It's better to test screenshots and static designs on these important users than a really buggy app.

Lastly, I strongly encourage you to switch from TestFlight to Hockey App. We made the transition after too many headaches with TestFlight.

I spend almost all of my time obsessing about attaining mobile nirvana and also built a website in the late 90s that provided personalized nutrition plans via a website to corporate employees so happy to talk to you in a call about what you're doing.

Excellent question! I've seen these issues firsthand (both myself using lean startup in my health 2.0 co., and with my clients) and I honestly don't feel there's a 'one size fits all' answer. Based on the info you've posted, I think you need to really understand what your main goal is. If it's for reversing disease, leave the B2B piece until you've worked the bugs out with your patient beta testers and make your product work best for them. Make it a great product. Get a community of users. Then bring to influencers and your potential B2B customers when it's working and has a user base that allows you to better demonstrate the value to that set of customers in the data, etc. Happy to chat further if you have other questions!