This image will not work on Rock64pro because it is based on a Rockchip RK3399, Rock64 has RK3328.
(In my opinion, Rock64pro is not a suitable platform for Volumio, but that is another story).
We do regular updates for Rock64, it is stable according to feedback here and on the pine64.org forum.
You should be able to update Rock64 when a new version is available, I personally have to admit not testing this very often.
I have not tested the Touchscreen plugin (which also supports plain HDMI, like a TV) as I have no use for it.
Perhaps someone else might chime in with info.
There is no login/ admin necessary, but you can use a remote ssh terminal connection access Volumio directly.
This can be used in case you want to do specific configuration, requires you to be confident with command line operations.
Indexing is done thru the UI, you need to configure settings for network storage (CIFS or NFS), also thru the UI.
With direct connected storage like USB sticks or drives this should work without automatically.
Not everything is kept in RAM, all configuration (including your music index) is kept on the SD or eMMC you booted from, but done in a way to minimize wear on your storage device.
USB Audio should be OK for most commercial devices.
If TEAC UD-501 is supported by the linux audio driver (google a little, I don’t know by heart), then Volumio will too.
You could buy the cheapest Rock64 as Volumio already works with 1Gb (and up).