Technology

If you're looking at getting a new (used) phone, I would suggest GrapheneOS (the most secure/private de-googled rom afaik).

You need a Pixel phone, the newer you get the longer you will keep getting software updates for the future (if you keep the phone past these many years of support, then I believe switching to a other rom will be required for security patches etc. Each phone is supported until Google stops supporting them I believe). You said you don't care about updates because you can keep it from connecting to the internet, but it's a plus anyways.

If you plan on never touching a google service, GrapheneOS allows for that (nothing google by default), but on the other hand, if you need google play, etc for banking apps or whatnot, they have that covered with Sandboxed Google Services (which you can run solely in another user profile on your phone for added privacy).

Anyways, I think GrapheneOS in a great option & their website has much more info if you'd like to continue hearing about it:

https://grapheneos.org/

p.s. you can check their website for how long different pixels will have continued support before (if) you get one (incase anyone else is reading this).

Ironically, pixels are best for de-googling (stock android that can be easily un- and re-locked)

You have to be careful to get a phone and model supported by one of the projects. Check all compatibility and install instructions before buying a phone. And if you need a manufacturer supplied unlock code, make sure the manufacturer still gives them out . Some will discontinue that service after a few years.

For graphene os you need one of the gogle devices - i've never tried it but i think its the one most people like.

lineageos supports more devices usually older.

I recently got lineageos working on sony experia xa2 - very happy with it. But to get there i had to go try like 6 computers before one of them sucessfully sent the bootloader unlock code over the ADB. For some reason usb is temperamental when doing stuff like that

It is a lot easier on really old stuff like samsung galaxy s3 or s4 if you can tolerate something that old. Maybe you'll lso end upon an old version of lineage.

Once you get the bootloader unlocked it is generally straightforward. but modern phones make that fist part awkward.

It depends on your definition of 'deGoogle'. You can disable the Google apps on most Android phones. They'll take up storage space, but won't run.

If you're getting a second-hand phone and want to completely deGoogle it, you can check if (1) the bootloader is unlockable and (2) custom ROMs are available online (e.g. Lineage OS compatible devices). In general, Xiaomi, Motorola and Pixel devices have unlockable bootloaders, but not all their models have custom ROMs.

If you have a phone with custom ROM support, pretty easy. I've been running LineageOS without GAPPS for like 5 years now. Most stuff just works, but to be fair, I am not using any of the cool kids apps like google pay or android auto.

Easy for manufacturers to deGoogle. Trivial.