Excellent review.

In all honesty I feel the idea is a lot better than the execution.  I had considered conducting this experiment myself but decided against it feeling it's not as safe, secure, or functional as an air-gapped Pi (or other PC) running an open-source OS such as Ubuntu.  My main concern with the setup is that you have to keep your seed phrases accessible or pull them out of hiding once in a while, which exposes them to significantly more risk than is necessary.  Conversely, an air-gapped PC can store your seeds relatively safely behind strong encryption, and multiple layers of encryption can be implemented.  Once you're seeds are paired with wallets they never need to be exposed again.

Theft of loss or theft of SeedSigner appears to pose slightly less risk, assuming is the thoroughly purged from the system.  I haven't audited the code, but I assume the seed is stored in ram while the device is in use.  If the OS uses or swap files to store the seed, it can retain the information unless it's overwritten with random bits as the device is being powered down.  I don't know about you, but none of that would really ease my concerns if the device was stolen.  Again, a thoroughly encrypted OS is likely to buy you more time to discover the loss of the device and move your funds.

One benefit to the experiment, however is that if you tire of it you have all you need to convert it into an air-gapped PC (with the exception that I would want a bigger screen.)

All in all, it looks like a really fun project whether you use it to store funds or not.