Word of advice: do not do this to any device that you actually depend on. Linux enthusiasm is all fine and fun, but this will kill most practical functionality of your device. I'd say try it out on a old phone you might have laying around but not on your daily driver.

If I'm not mistaken, I believe the 2018 J3 has a locked bootloader. The fact that I can't find even a SINGLE custom ROM on XDA for this model means it's highly likely that the bootloader is locked, and/or the device isn't dev friendly (no kernel sources available etc).

so I guess doing the same on my smartphone wouldn't be too hard.

Mate, you've no idea... Smartphones are a completely different ball game to desktops. You could try and compile your distro, but without the kernel sources and drivers for your specific model, nothings gonna work. You won't even be able to boot the damn thing. And even if you did have those, it's going to take a LOT of effort just to get basic OS functionality working. Forget getting actual phone stuff working, like making calls etc - that's next to impossible. Even large projects like PostmarketOS struggle to get basic functionality going even on dev-friendly phones.

But you can stop dreaming about all the above if you can't even unlock the bootloader.

Basically, what all this means is that there's no point wasting your time on the J3. Stop right now and don't waste any further time on this.

If you'd really like to run GrapheneOS / Linux on your phone, your best option is to sell your J3, and get a used Google Pixel from Swappa/eBay or something.

No. None of it is worth the effort. None of it works well, you'll fight with payments, and shit is going to be buggy and unless you really want to learn you don't sound too technical.

I daily drive GNU/Linux on my Phone (PinePhone Pro), I would say it usable, but Android is way more practical.

Also not a lot of devices can run GNU/Linux and have fully functional HW due to missing drivers. And only 3 phones of them can run mainline kernel.

Waydroid works fine, I use it for banking apps, but it's a bit slow on my specific device.

For a spare phone if you want to just tinker and have fun, go ahead. For a main devicez just. Don't. It is very very alpha quality software.

Ok I'd cut through all of this and focus on your needs: phone with no spying, can't find rom. I assume you mean your phone isn't supported for graphene, lineage, or calyxos, or any others I missed?

Easiest way: buy a phone that is supported for the privacy minded os you want.

Phones aren't like old laptops. They're a different architecture entirely, and the hardware is often somewhat custom to the device. Building an image that would even boot on it would be a challenge, much less getting stuff like the touch-screen etc working

I guess doing the same on my smartphone wouldn't be too hard

Snigger

Have you installed a custom ROM on it? If not, you definitely don't have the skills for this. If so, have you built your own ROM for it? If not, do that so you learn how it works in a predictable environment. Then port something existing to it, like UBPorts. Only after you do all of that and probably a lot more should you attempt to effectively develop your own distro on hostile hardware.

As others are saying, that's not really an option unless you're really dedicated. IF it has an unlockable bootloader you could technically get to compiling and tinkering to get everything built, but in order to get a phone all set you'd need to get the right drivers and do a whole lot of tinkering (like full time job levels of building and tinkering) kind of deal to get it built. Phone's aren't so plug and play like computers.

If you there's no rom support and/or a permanently locked bootloader but you want an OS without x y and z you can always try to fire up ADB bridge and disable stuff. You could also accomplish the same by rooting, though it's a bit of a security risk (though not as overblown as some people say IMO).

I just bought a OnePlus 6 to test out mobile Linux and it's not there yet. Firefox it a pain to use and it doesn't auto rotate either. So far it's been good to read manga on and.... Ye that's about it. Camera doesn't work on it and the UI still isn't the best. I haven't used KDE's DE for phones yet but I've used phosh and now I'm using gnome mobile and so far gnome mobile is a lot better but still buggy. I'm excited for the future development of it but with how locked down phones are it's a bleak future