The iFi power supply comes with a bunch of adapters to fit different items. Included is one that adapts its output to miniUSB as used on the RPi3. If you are using an RPi4, I don't know.
A HAT card plugs into the RPi to give either digital or analog output. The case encloses both the RPi and the HAT card.
By pre-assembled, I meant this thing: https://www.allo.com/sparky/digione-player.html
. I prefer the aluminum case, which includes internal shielding.
You also have to specify an operating system, which also provides player and other software. The OS I've got to work without a lot of fuss is Volumio. It has a quirk or two, but it's regularly updated and does the job. You will not need to attach a monitor and keyboard to configure Volumio, as it's designed for Web configuration; not true of all of the OS options.
That solution outputs digital, so you could feed it into your AVR or into an external DAC. For analog output, you would use a DAC HAT card instead of the digital-output one. I have not tried the Allo or other DAC cards (except the HiFiBerry).
However, be aware that a HAT card typically requires a special case for itself and the RPi, because the connector positions are not standardized. That's one advantage of pre-assembled; you automatically get the correct case.
Allo offers this as a pre-assembled analog-out solution: https://www.allo.com/sparky/katana-player.html
I have never seen, never mind heard, one, but I'm including it as an example of what is available.
Hope that helps.
P.S. The iFi power supply made a surprising difference in my setup. If you have an RPi setup already but are using the stock power supply, the first thing I'd recommend is trying the iFi iPower. It is not terribly costly and might be all you need to get suitable performance.