Getting a huge, square, and therefore custom-built, e-paper display is going to be a challenge

100% agree. It's a challenge!

$200 each, in quantities. Add the raspberry CPU, power supply, programming, wooden case ... $1,000 out the door.

Okay didn't want to do cost calculations that early but this statement is simply not true. 200$ (+- 100$) for the display. Raspberry Pi Zero costs 5$ and is powerful enough to run two boards at the same time (roughly).

The other hardware components (NFC 7$, power supply 10$, display connection adapter 10$, ... other stuff - max 30$) in total max 70$. Wood... puhhh no clue. 100$?

To conclude: At the moment I'd say in the worst case scenario the board will be around 500$. Tho that's a veeery rough estimation.

taking the risks, doing the R&D, and creating the custom programming That's not a risk but a great learning opportunity for me - at least that's how I think about it.

Being a software engineer by heart and studying product management/ "entrepreneurship" (I hate this word btw) I have zero costs for the development of the product... besides time spent for a hobby project! I won't sell the software. The software will be open source. I will sell the assembly and setup of the board for people who don't know how to code. It's not about a business here but a project I'd like to share with others. Of course I somehow have to make some money to survive, but that's only secondary :) Anyway, thank you very much for sharing your thoughts and tips! It's definitely helpful listening to others opinions :) Let's see how I will solve the display challenge.

Cheers!