HT and 2 channel in same room? 1 or 2 systems?

IF you have the space, two systems without a doubt. Movies are radio with pictures.

Large screens can be covered with a quilt for HiFi. Projectors suck compared to OLED, well actually compared to any modern direct view.

Use subs that have dual inputs for HiFi and HT e.g. Martin Logan X series, Anthem, etc. Get subs that have full control: Multi slope and continuously variable XO, continuous phase, polarity inversion.

The DAC in the AV amp simply isn't up to it

Interesting, I added the Sony Signature TAZH1ES DAC/headphone amp/pre for the same reason. I tried it as a preamp in two channel mode but prefer the use of the room correction of my Marantz processor. Now I just run it via the RCA connection into the processor. Another benefit of the outboard dac approach is that it is easier to upgrade a dac then swap out an entire processor or receiver.

Projectors suck compared to OLED

No, they both have advantages. I found a good way to take advantage of Dolby Vision on my non DV PJ was to use a Zidoo device. The DV engine in the Zidoo is able to be read with the HDR in my PJ. Even DVD's look awesome (maybe not OLED awesome but how much is a 120 inch OLED? LOL)

I have 2 channel and AV 5.1 in my low profile credenza I have a Oppo BD Blu-ray player that has RCA sub out that goes to my two Subs and HDMI that goes to my Emotiva XMC-2 AV preamp Also BNC digital out to my Exogal Comet Plus DAC From my DAC l go XRL out to my tube ARC LS15 preamp and from there to my VAC tube amp So when l want to listen to my 2 channel rig l select coaxial out on my OPPO to my DAC When l want to listen to my 5.1 with the OPPO, Firestick, LG OLED or Xbox l select the output on my Emotiva XMC-2 I use the XRL from my Emotiva to my subs also I have two set of speakers Towers with built in powered subs and Maggie’s and use both in my 2 channel and 5.1

I went through this when I added video to an earlier system and again when I moved house and gear to a new location. Some of my conclusions:

1 - you need main speakers that are dual purpose, i.e. that have the range and sound good with either pure audio or video, which means that they have to have decent bass output - bookshelf speakers need not apply

2 - there is no such thing as a really good surround processor that will also do a truly great job in the pure audio system, so don't try.That means buying a good quality AV preamp or receiver and having a separate amplification chain for audio and video

I bought an Marantz AV preamp that does a very good job on movie sound but does not sound as good as my all tube audio chain.

3 - you can use the same power amp for thee main speakers if you like, or you can do what I did - use a different amp. That way I reserve tube life for music and use an old Rowland 5 solid state amp for AV - all I have to do is pull the speaker leads out of the tube amp and plug them into th Rowland

4. if you have a big room and your speakers aren't exceptionally good on bass, you will want some bass reinforcement, particularly as movie sound tracks use the impact of bass to stun their audience into admiration. That means a separate subwoofer (I use a pair of matched powered Hsu units that give good clean low bass, a fairly rare thing in reasonably priced subs.)

5. The AV preamp can feed a separate multi-channel power amp for surround speakers. You want decent speakers (I used all Vandersteen). Audition in the room to see whether or not you need a center channel speaker - it turned out that I didn't as my main speakers produced a better phantom centre channel than a centre speaker I had bought (and subsequently sold on)

6. - Pay someone with the specialized sound equipment to set up your speakers and room - it will ay of in the pure audio system as well.  Mark main speaker position with tape or marks in case you ever have to temporarily move them.  The analysis will also point out any slap echos and room flaws that you may want to address then or later - the solution may not be too expensive - heavy curtains you can close for movies, for instance can tame some problems.

If your main speaker placement in the room is wide enough that you can use a regular screen, you can buy a regular screen (mine is a 10 foot diagonal that is powered and winds up behind a false beam in the room when not in use.  If speaker placement dictates, you may need to locate the screen in front of the speakers (i.e. if they are placed closer together than the screen dimensions) in which case you have to but an acoustically transparent screen material (they aren't actually sonically transparent - they usually lose 1-2 dB but that can be compensated for when the sound guys sets it all up).

My set up can be seen at