At this point, I believe the most effective ways one can help with this is:
(1) advocacy - it's slow and difficult, but having people at least agree / be familiar with the idea that closed stuff is bad is a good first step.
Open ecosystems can't work for the general public if it's trapped in closed networks that won't work on anything else than the two big mobile operating systems, so making people start using open chat apps and such will help a lot. It'll take years, but so be it. It's worth it I think.
(2) helping improve the more open stuff.
I think Linux mobile for instance is a potentially viable alternative in the medium term for at least the basic use cases: Calls, SMS, GPS / Maps, Signal, photos. All this has no reason not to work with some polish. I daily drove Linux mobile 4 years ago for a year. The main thing I'm missing is good hardware for it, and a lot of polish but nothing impossible. Yeah, indeed, no payment with the phone (Google Pay / Apple Pay). But it's still possible to use the physical cards and not use the phone for this.
A locked-down Android is pointless.
I make relatively decent money by our standards, and I wouldn't even think about dropping $700-1000 on a phone (which isn't even officially sold or supported over here). For the vast majority of people it's their whole income over 2-4 months. I don't know or care how much you make, let's say it's $10k per month. Imagine if you had to pay $20-40k for a phone which is good for maybe 5-8 years.
And most of the world is like that.