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.