Ditto on the Fusion. At the NY boat show last year all the high end boats were running Fusion gear (see also: Lumitec LED lighting, Isotherm refrigerators, and Kenyon stoves lol).
There are pricier models slotted above the RA70 / RA70N that have true dual zone, but I don't think they are DIN sized. As mentioned above - the "N" in RA70N adds NMEA2000 control. You don't need a Garmin screen specifically (although Garmin does own Fusion), any NMEA2000 compatible multifunctional display will run the Fusion Link media control app. Here's a video of a SIMRAD MFD running Fusion Link, for example:
Since you're replacing a DIN sized unit, look for like kind if you want to avoid mucking about with the mounting hole size. Lots of options and sizes there, including totally hidden base units with wireless, NMEA, and bluetooth control.
For speakers, Fusion's are quite good. Best I've probably come across in terms of volume and clarity are the JL M6 series 6.5's, but they run something like $500 a pair with LED lighting, $430 without. They also have an M3 series which handles 60 watts RMS instead of 75, at far less cost ($229 at Crutchfield, currently). I'd think these would be a great option. JL is quality stuff.
On my 3300, I replaced my rotted out 17 year old Clarion 6.5s in the radar arch with a new $100-ish pair from Kicker, and even they were a massive improvement. I have an RA70N slated to go in that boat this summer, replacing the existing old JBL CD player with CD changer. I currently run an RA70 on my 20 foot runabout (no "N" needed since it's a lake boat with no MFD).