Sadly it's pretty hard. I have had a great experience with Lineage OS for several years now. Installing a custom ROM is not that hard imo, but it is scary to most and few flagship devices allow it. But I think more competition will be the key to more freedom and openness, and that is coming: Google is being pressured to embrace 3rd party app stores, and Apple is being pressured to allow side loading in Europe. Also things like RISC-V and the growing open source hardware movement are shaking things up. The death of Moore's law will also bring more competition into chip design in the coming decade which could help to establish more open standards and options in the market.