I can confirm that a 2016 WRX rim, 18x8.5 with a 245/45/18 fitted will fit under the boot floor with no clearance issues.
A 130mm long m8 bolt and a plate/washer will secure it down.
It's this one here https://www.wavetech.co.nz/REV-TOY-2/
It puts out a max of 6V at 3A.
Currently, I've just taken the power from the 12V cigarette lighter socket, as the rest of the wiring from all of the Japanese aftermarket accessories is all over the place.
Hey @HaydenHillas I've just spent most of today installing a new Sony head unit into my 2016 Levorg, including powering the MFD unit.
I used a 12V to 6V converter to power it (as well as the reversing camera) and removed the cable from the old Panasonic head unit that was plugged into the MFD...
While I've been looking around, I have found that the Panasonic CN-LR710DFA appears to be a dealer option replacement head unit, as per this listing.
https://www.croooober.com/en/item/cparts/00000173/Memory-Navigation-digital/14831156/Panasonic-CN-LR710DFA-Subaru-Dealer-Option-Navi
I have also recently started looking into replacing a Panasonic Strada CN-LR710DFA, everything appears to be the exact same as yours.
You can probably run that module on 6V with no issues, you might find that it ends up dropping from 6.4V to 6V under load anyway.