That's true for literally anything, not just IoT security and privacy. I mean, even for highly technical users, one can't do everything from scratch, nor even check and control every single aspect: you gotta trust the the computer hardware or OS you're using isn't backdoored, you gotta trust the people that built the place you live in didn't put half the rebar actually needed or wired the whole thing backwards or with thinner-than-required wires, you gotta trust that the food you eat isn't going to make you sick...

Same for HASS, one could delegate trust to a specialist that would install a HA Green or Yellow box for them, just as they do for electrical wiring. HA is only "third party" because the IoT place lacks standards but is in essence no different than wiring stuff from different vendors, where "myriads of decentralised solutions" exist only because of standards, and for which decentralisation essentially means everyone is a third party to everyone else.

So I don't think dismissing HASS as third party is fair, and wiring IoT with virtual wires is no different than wiring a breaker box. If you don't know how to do it it can be dangerous, and so you delegate and trust someone to do their job properly.

The problem is that approximately NONE of the commercial vendors are in any way trustworthy. They're really pushing hard the degree of abuse they inflict on the customers, and social immunity takes long time to build.

The ultimate solution IMO is to have people trust in people they can actually trust - that is, make the third parties local. A partner, a kid, a neighbor, a small company servicing the local community and physically located in it. At this scale, trust can be managed through tried-and-true social techniques humans are innately good at, and have successfully used for many thousands of years. This is how you make most of the tech industry and adjacent problems go away.