Comment by jeroenhd

Sailfish has a Linux-based OS with some Android compatibility.

Ubuntu Touch has left behind UBPorts which uses Android's hardware layer to provide a usable UI on existing Android devices.

There's the Librem phone with an open source stack, though its hardware doesn't come close to a cheap Android phone these days.

There are projects like PostmarketOS which work to get mainline Linux on phones. From there, you can run Linux on phones through desktop environments such as Phosh and Plasma Mobile which are touch optimized.

FirefoxOS died but was forked into KaiOS, though the modern iteration of KaiOS is an Android fork.

Samsung has TizenOS but I don't think any phones run it anymore. It's probably the closest equivalent to Android in the way it has been developed.

If you want to throw money at something, Sailfish may be the project to keep an eye on. All the other volunteer-run operating systems are worth donating to, of course, but Sailfish is actually trying to be a real third option rather than an open source proof of concept or alternative for shitty vendors stopping updates.