Buy a phone with an unlocked/unlockable bootloader, and use custom ROMs to stay up to date long after the manufacturer has stopped caring about support. Unlocked phones seem few and far between nowadays, but there's still some. Here's another not-so-subtle recommendation for the Google Pixel line.

I've been using a Moto X4 (8 years old!) with LineageOS for 6-7 years. I'll probably get an open box (for a discount) Pixel soon, and probably put GrapheneOS on it.