We have a Yoto here as well, for our six-year-old.
The concept is great - RFID as a replacement for cassette audiobooks (with fewer storage limitations!).
I do wish it integrated better with sources of free audiobooks. The Libby app gets us access to a lot of audiobooks through the public library, many of which are not even available for purchase through the Yoto player. We can only use it to play them for him as a Bluetooth speaker from our phones, which removes a lot of the utility of the player (he can't navigate chapters, we can't set a sleep timer, we can't use our phones for other things).
The concept is great though and the specific product, walled content garden notwithstanding, has been a net win for us.
The make your own cards are really nice for this. We bought a bunch of them and you can add any mp3s you want onto them. We even print stickers to put on the front.
The Yoto system actively encourages you to buy 'blank' cards to fill with your own content, and the process is relatively simple. Simply remove the DRM from the borrowed media, (convert to an appropriate format if required), then upload to the card. Wipe your card whenever you borrow a new audio book from the libarary for a clear conscience. yt-dlp is also a great source of content.