Various websites suck in one browser or the other or simply don’t work in more than one single browser. We’re not that far away from the days when Internet Explorer (IE) was the only thing that loaded a site (often for something work-related… groan)

That said, if you need Chrom(e/ium) and want a non-data-sucking version, I think Ungoogled Chromium is your best bet currently.

For the Firefox side of things, there are already several forks that aim to do things differently/better. Floorp is one I see recommended regularly. There seem to be a larger number of Firefox forks focusing on security/privacy than Google forks, but this is the most well-regarded from my research.

Simultaneous post-enshittification from both Chrome/Chromium and Firefox is probably (hopefully) leading towards more active development/contribution to these (and other) forks!

There are cases other than what you mentioned where the carrier refuses to unlock. During 2021, AT&T forced a phase out of 3G and provided allowed customers to get a free upgrade phone in the process. As a prepay customer, I took the phone but jumped ship to a different carrier. I tried to get them to unlock the phone earlier this year and they refused. There was no contract/lease involved, but they didn’t like that I didn’t remain a customer with them for (I think it was) 180 days after that.