Doesn’t sound like a bad idea, but there might be some better options depending on the type of media you plan on using it for.

If you’re mostly gonna use it for listening to music, you should look into a PMP/DAP. I’ve been wanting to buy one for a while and the FiiO players seem to get a lot of love. The higher-end ones usually run Android but they’ll probably be more expensive than an Xperia Z21 Compact. They’re designed with audio as the main focus though. There are also a lot of cheaper options that’ll give you a better audio experience than an Android phone—the FiiO Echo Mini is only $70 USD, but I don’t think there’s much more you can do with them other than listen to music.

I haven’t looked into video playback at all, but the higher-end ones usually have screens comparable to the Z21 Compact, and I wouldn’t be surprised if some have better displays. A screen as small as the ones you’ll find on the Z21 or an Android PMP isn’t gonna offer that great of a video experience no matter how good the resolution or refresh rate is though.

Another bonus with the higher-end PMPs is some of them can be used as a DAC, which is pretty effin rad imo.

FiiO also seems to have a pretty active modding community and some custom ROMs are available too. And if the audio quality on Xperia devices is as trash as it was on the Pixel 6 running LineageOS I was using up until last year then you’re definitely gonna want to root it (if possible) and install ViPER4Android.

Another option is Rockbox. If you have a compatible MP3 player lying around, might be worth trying that before making a decision.

If you can afford to buy it and possibly brick it, then sure, it’s a good idea.

“installing operating systems” on a PC or Mac is usually just booting from a USB and going through a GUI. If you want to install custom ROMs to an Android device, you need to get comfortable with a few things:

• Using the command line
• Flashing recovery images
• Flashing custom ROMs

What you need to do first (before you can install a custom ROM) is replacing the stock recovery on the phone, the environment that you boot to in order to do certain things like wiping system data and of course, flashing ROMs. For LineageOS, the recommended one is called “TWRP”, and it is probably the best. There’s a guide on the TWRP website for the XZ1 compact here.

Once you have the custom recovery in place, you would need to transfer the custom ROM to the phone, then follow the instructions on the LineageOS post to actually flash the ROM to your phone.
LineageOS 17/18/19 for Sony Xperia XZ1 Compact

This is a simplified overview, and is in no means a complete guide. I highly suggest watching some YouTube videos, and reading some of the guide posts over on the XDA forums before committing to this project.

Hope that helps!

I was thinking of doing the same. The LOS17 build on XDA seems to be maintained and used by its dev and has working VoLTE + stock camera, so it seems pretty good software wise. My only concern is the battery which seems pretty hard to replace or buy… thus I haven’t yet done it.

I think its a cool little project. If you like tinkering and screwing around with tech just go for it. But there are few things which come to my mind. Dont take them as advice against it more like pointers and things to think about.

1. As much as i miss them as a common feature in todays phones. Headjacks on phones were most of the time kind of shit. I had 30€ mp3 players which provided better audio quality than the Z1 compact i had at the time. Idk if the XZ1 got way better? I think the issue is that most phones had terrible D/A converters. But dont quote me on that. But this is only a concern if you have high standards for audio quality for wired headphones. If you use something with bluetooth the point becomes irrelevant. LG V series phones were really good in that aspect and also pleased the audiophiles. They are much bigger bricks though. If you want to use it mostly for audio a used mp3 player might also be a way to go because they can be even smaller.

2. Installing customroms isnt that hard if you can follow a guide and you are able to use a searchmachine on your own. But its still easy to mess up. I messed up quite a lot over the last 15 years. But atleast my mistakes were always recoverable. Just dont rush things. Take your time. Read up as much as you can before you start the process. Biggest issue i see that there isnt an official build for lineage. With that also no official documentation/install guide which you could just follow religiously. My experience is that those builds provided on the XDA forums come with as little information as possible. They just expect you to know your shit? With that often comes a bit of ambiguity because you’ll probably find different answers for the issues you search for. It can slow things down a bit and involves trial and error. But in the end i always managed to get things happen and i dont have any professional background in tech.

3. You might want to read up on how easy it is to swap the battery. These phones and with that their batteries are getting old. But in my experience there is for almost every phone a video teardown on youtube.

4. Biggest battery drainers are the antennas. Without a simcard, no wifi, etc phones can last ages. I use my old phone for a similar usecase as yours. It doesnt get too much use but with that i need to charge it like maybe every 3 weeks or so. Its crazy.

5. If you want to use the camera you might want read up on if the XZ1 is still a sony phone were the camera quality drops significantly with a customrom. They changed that some years ago. But idk when exactly.

6. If you want to use it as an offline device mostly for audio anyway(just assuming, idk) maybe just try out the stockrom a bit. It could fullfil your needs already perfectly.

7. Check out Emulators. That snapdragon 835 should play anything easily up to the Dreamcast and PSP maybe even some lighter PS2 and Gamecube Games.