Hi Antonia and welcome.
I saw your thread on Tapeheads and as you can see we have a long running thread here about the delightful faff and expense that is R2R.
I think @murrayjohnson has already kind of asked what I was going to ask you on THs, which is who do you think is your target market, how big do you estimate it to be, and what do you think they would like (and be willing to pay for most crucially).
I think 4T at 2 speeds is the most attractive if you’re looking to serve the market like me that wants to listen to pre-recorded tapes and make recordings of hi res digital and vinyl for playback.
I suspect 8T is what musicians and recording engineers ‘might’ want.
Depending on which market you’re after, you will have to probably have to answer a pretty basic question, which is why should I buy your product versus a vintage unit?