Your plans will probably change a couple of times before you make your final decision, certainly mine did, but I agree with @
soundman98 in that having a DSP that you don't have to continually run back and forth to make adjustments is a much better solution. DSP is a godsend to car audio nowadays. All of the equipment in my current setup is aftermarket, including my Pioneer DEX-P99RS head unit which has four-way active network capabilities and built in DSP that automatically sets itself up compensating as best it can for the listening environment. It just needs a bit of manual adjustment afterwards.
For highs, just yesterday I installed some Kicker pod-mount 3/4" tweeters mounting them to the dash speaker grills and have them bandpassed between 5kHz to 16kHz at 24db/octave on both ends. See here,
Attachment 206056
Attachment 206057
For mid-range, I have some Dayton Audio 3-1/2" drivers in the factory dash locations which are bandpassed between 2kHz and 5kHz at 24db/octave on both ends.
For mid-bass in the doors, I have some Dayton Audio 6-1/2" drivers bandpassed between 100Hz and 2kHz at 24db/octave on both ends.
My 12" first generation Orion, dual two-ohm voice coil HCCA subwoofer is mounted in a custom-built sealed enclosure handling the low frequencies bandpassed between 20 Hz and 100Hz at 24db/octave on both ends.
All of these components are amplified using Cerwin Vega Stealth Bomber amplifiers. The B54 four-channel amp is handling the tweeters and dash midrange drivers, the B52 two-channel amp is handling the door mid-bass drivers, and the B51 mono-block amplifier is handling the subwoofer.
I have left the rear speakers disconnected in this setup, as I really don't need them.
It sounds fantastic. I'm not going for SPL on this build, this is a sound quality build with Boom Mat in the inner and outer door skins for sound deadening.
Coming back to your situation, have you looked at Audio Control's amplifiers with their DSP built in? Here is their six-channel amplifier which may be of interest to you,
Attachment 206058
This will accept speaker-level inputs such as the ones from your head unit, which would substitute for the line output converter that you mentioned earlier, process the input, which would substitute for the digital EQ that you also mentioned earlier and give you much more control over the sound and would also add 125 watts per channel on six outputs, with a separate processed subwoofer line output. This would simplify your setup a great deal. You could amplify all of your front speakers and send the processed subwoofer signal to another amp for a subwoofer in the spare tire well, and you can set it all up with a laptop or phone app.