Simplest possible, least invasive, most secure thing I can think of: QR code on the router with the CA cert of the router. Open cert manager app on laptop/phone, scan QR code, import CA cert. Comms are now secure (assuming nobody replaced the sticker).
The crazy thing? There is already two WiFi QR code standards, but they do not include the CA cert. There's a "Wi-Fi Easy Connect" standard that is intended to secure the network for an enterprise, and there's a random Java QR code library that made their own standard for just encoding an access point and WPA shared key (and Android and iOS both adopted it, so now it's a de-facto standard).
End-user security wasn't a consideration for either of them. With the former they only cared about protecting the enterprise network, and with the latter they just wanted to make it easier to get onto a non-Enterprise network. The user still has to fend for themselves once they're on the network.