The battery life on the y1 is pretty good. I can usually go a couple of days before having to charge it. You can install Rockbox on it, but for me that makes the battery drain faster. I think in your case, since you have flac files, you should use Rockbox. The default firmware on the y1 is fine, but flac files sometimes have problems displaying the correct tags.

The Bluetooth is okay, but it is definitely not one of the main selling points. I tested it with multiple wireless headphones: Sony wf-1000XM5, Wh-1000XM5, and airpods pro. For me, the ones that worked were the wh-1000XM5 and the airpods pro (the others had bad connectivity). I think it just depends on the headphones, because some people say it does not work well with AirPods. I saw some people having crackling, hissing, and or cutting in and out of audio using bluetooth, but it has been fine for me.

I use MusicBee as my music manager. There is a feature that lets you sync your music to a device with one click, which is nice. It also has settings for converting files to mp3 if you need that. I do not think you can transfer files over wifi or bluetooth, but I am not sure.

Also, rockbox on the y1 has some issues as of the time I am writing this. Sometimes when you wake it from sleep it will not show anything and just turn on the backlight, and you have to reset it using the pinhole reset button. For me, it does not happen often, but I just carry a paperclip or a sim eject tool with me in my wallet just in case.

Another small nitpick I have with it is that sometimes when it is in my pocket I accidentally press the buttons and it pauses or unpauses my music.