For what it's worth, I got the ZX507 first and then found a good deal on a ZX300 later on to see if the battery life + custom firmware would be better than the ZX507 overall. But the ZX507 can still sound really good, just not as flexible as all the different firmwares and regions you can use on the ZX300.
The ZX507 has the convenience of USB-C, and with about 10 minutes of deleting/disabling/opting out of apps you can remove some of the bloat of the Android OS. The ZX507 ends up with a nicer screen and IMO a slightly better UI, managing bookmarks is nice, being able to type/search through a large library is nice, more preset options, and so on.
For example, if I'm listening to a track, I can tap on the artist's name and in one tap, navigate to other albums by that artist or the rest of the songs in the album the song I'm listening to is from. Bookmarks are also not my favorite (I would much rather prefer being able to add to the existing play queue, Play next, play at the bottom of the queue, like on some Astell Kern DAPs, for example) -- but on the ZX507 you can make a bunch of lists, rename them, and view them vertically in a list rather than the ZX300 where you have to tap through them horizontally like pages.
The screen on the ZX507 is just bigger and easier to use, although it isn't matte, and many might find stuck subpixels (I have about 5 along the edges/bottom of the screen that I can notice if I have it on full brightness with a black background and tilt the device around). But it's much easier to use outdoors given how bright it can get.
I mostly use mine offline/downloaded music but it is nice once in a while (like, say every Friday when new music drops) to preview/stream lossless stuff and discover new music before committing to buying, all on the same device. With stuff like Apple Music ending up with a no-cost-to-upgrade lossless option it sort of added value to my ZX507 as it's another app I can stream lossless on.
The ZX507 isn't without its faults, though, streaming battery life isn't that great, but unless you want to use it for like, the entire day, or just absolutely hate plugging things in to charge, it seems to last long enough for my use cases. I can see why it would be annoying to have to turn it off because it takes some time, but it definitely doesn't take as long to boot and rebuild a database like NW-A55 and ZX300 so it's not too bad. There are also auto-off features you can set to help conserve standby battery on the ZX507 thanks to its Android OS.
Anyway, I'm in the same boat as you, but the UX of the ZX300 and ZX507 is still pretty different so I am hanging on to them both for now. The W1MA/Z might be due for an upgrade soon, but I fear it will go USB-C + Android just to stay competitive in a modern market. To sum it up, there are some improvements IMO in the ZX507 in the Walkman app alone, and the downsides of Android aren't sooo bad if you take the time to optimize the settings and turn off what you don't need/use. But there's the added flexibility of being on Android in case there are apps in the future you'd find value from. hope that helps
edit: lol like half an hour after posting this my ZX507 crashed/shut off while listening to the walkman app. so there's that
