Hi @Fullitarko
No problem! That's great you would like to write a post on this. Please send me an email to robert@bela.io and we can discuss in more details there. Looking forward to sharing this with the wider Bela community.
Regarding selling a product with a Bela inside it, sure, you can do this without any problems. There's some more info on our website: https://bela.io/about#making-products-with-bela
In terms of using Bela Mini for the Sousaphone, the CPU performance is exactly the same as the bigger Bela so will work no problem.
We do offer quantity discounts if people are buying a larger amount from us at once. Typically these start when people are buying 10 or more boards but if you let us know what you have in mind we'd be happy to discuss over email. We could consider selling Minis without SD and cables for example.
Cheers!