You need a router that allows you to configure strong policies, such as client isolation, what data can flow between VLANs, etc.
More broadly you cannot solve every with technology. The most effective route would be to simply outlaw such analytics without informed consent. This is basically what the GDPR does, but it takes a while before enough companies get fined before the industry understands. That said, the last few years, products sold in the EU are starting to get toggles to request analytics that are _off_ by default, etc.
If the device does anything with network, other devices can likely sniff a lot of information about it with high degree of accuracy based on how they react to certain network probes.
and my point was that you can prevent this by using e.g. WiFi client isolation and isolating VLANs (while still using the 'smart' features of said device).
I am not sure how your comment is a reaction to what I said?
The problem is that most ISP (modem+)routers are bottom of the barrel devices that do not allow users to configure such features. Even worse, some ISPs also scan the user's network and sell data to analytics companies [1]. This is a very good reason to always use your own router. If you want something with an Apple-like experience (mostly), Unifi gear is quite good. If you want something open source, then a router with something like OpenWrt or OPNsense + an access point with OpenWrt will do the job.
[1] https://tweakers.net/nieuws/245620/odido-router-stuurde-anal...