Helium is a Chromium based browser that will still support ad blockers, whose devs (unlile the CEO of Brave) seem like good peeps - https://helium.computer/

Besides Librewolf, Waterfox is also a good Firefox/Gecko basd option - https://waterfox.com/

The best general resource for instructions on how to unlock your phone’s bootloader and flash alternate AOSP ROM’s or non-Android OS’s is https://xdaforums.com/

Please note that not all carriers and oem’s allow you to unlock the bootloader though, so choose your device carefully for this.

You will also most likely need a PC (desktop or laptop) with adb & fastboot on it. These are apps used in the terminal, but you only need to copy and paste a few commands into them to use it.

If you have a Google Pixel then best option is Graphene - https://grapheneos.org/

For other devices you can use a “degoogled” Android ROM and get apps from the open source F-Droid app store - https://f-droid.org/

Some choices for this are:

Lineage - https://lineageos.org/

crDroid - https://crdroid.net/

/e/os - https://e.foundation/e-os/

Iode - https://iode.tech/iodeos/

OR use a a true alternative mobile OS. Options for this are:

Ubuntu Touch - https://www.ubuntu-touch.io/

Sailfish OS - https://sailfishos.org/

Mobian - https://mobian-project.org/

Postmarket OS - https://postmarketos.org/

Plasma Mobile - https://plasma-mobile.org/

Droidian - https://droidian.org/

You can also purchase devices with alternative OS’s already preinstalled from:

Volla - https://volla.online/en/devices/

Jolla - https://jolla.com/

Fairphone - https://fairphone.com/

Murena - https://murena.com/

Furilabs - https://furilabs.com/

Brax - https://www.braxtech.net/