Internal CAs and self signed certificates are different. You can still generate a CA, sign your certificates, import your own CA into your phone and have that verify your certificates. You don't need Letsencrypt. But you'll learn in time.
No non-sophisticated user is able to run their own local CA, and that's why their NAS, IoT setup etc. all run over HTTP only, which in turn has implications for available web APIs (thanks to "secure origin only" policies and no exemption for local IPs/zeroconf domains) and many other things.
It also doesn't work for at least modern Android apps, since Android no longer makes user-provided CA certificates available to (non-browser) apps anymore, I believe, unless they're compiled with a special debugging parameter. On iOS it's still possible, but I'm not sure how long it's going to stay that way.
"Openssl how to set up a CA" > First link fully explains it https://arminreiter.com/2022/01/create-your-own-certificate-...
"How to import CA into iPhone" > First link fully explains it https://www.ibm.com/docs/en/mpf/7.1.0?topic=certificates-ins...
"Android app customize trusted CAs" > First link fully explains it https://developer.android.com/privacy-and-security/security-...
The barrier to entry on PKI isn't that it's hard, it seems to be that people just can't be bothered, PKI is among the most google-able tech processes out there
You're completely out of touch with the majority of the userbase of these products if you think even one in 10 NAS users will set up their own CA using OpenSSL (in a secure way that doesn't expose themselves to being MITMed even on public sites such as that of their bank down the road).
In an ideal world they would load a letencrypt certificate and set up the tooling required to automatically pull down a new one when required.
A NAS company owned CA doesn't offer much of a benefit directly for the plug-n-play users, but it's still better than just a self signed cert, and for people who care about their security even a little bit it can significantly protection.
Most Plug-n-Play NAS solutions will integrate with a web api and/or an app, and it's more common than it should be that NAS'es are exposed to the internet.
Once you control both the NAS and it's clients, there's absolutely no reason not to preload a complete PKI implementation. Even just an installation app which loaded the chain onto any device you wanted to interact with the NAS would be sufficient.
If NAS'es are intended for non-technical people, then any NAS sold should be secure by default.
Requiring users to install a globally trusted CA is a disaster from a security point of view (now my NAS vendor or anyone that hacks them can pose as google.com!), and for this reason doesn’t even work with modern Android apps anymore, for example.
But if you want "secure by default" then neither one is acceptable.
Somewhere you still need to trust a self-signed certificate.