New Anthem AVM 70 and MRX 740 difference in processing

So you already have a multichannel power amp?
I have been told by a few people at shows, none of whom was trying to sell me anything, that the processor has better parts and sounds better than the AVR.  I was in your position and opted for the AVR without hearing the processor, but at that time, a few months pre covid, the difference was more like $2500

I have pretty decent speakers in my HT systems, but I opted for the Anthem Receiver in both instead of separates.  In one system I was replacing another AVR, but in the other system I was using a Parasound 5 channel amp with an older Marantz pre pro..  I retained the amp in the basement system and I'm using 
the Anthem MX 570 there just as a preamp, and compared to the old Marantz the improvement is huge.  I suspect the Room Processing has a lot to do with said improvement, and that it would have been money poorly spent to go with separates.
   In your case, since you yourself have a poor opinion of your speakers, which after all are the most important part of any system, it surely would be money wasted to go with separates.  You also state that the main use will be for video and not Music, which again means that (IMO) having a nice system is less important.  With your speakers you won't be able to get very immersive movie sound

I think the Anthem 520 would drive the rear speakers.  It should be easy enough to insert the 520 as a Preamp, then reattach the speakers to the 520 and do some comparisons.  If you have an Anthem Dealer that will let you borrow the MX 520 for a weekend that would be ideal.  Rears ideally should be for ambiance only in music.
If you can’t demo, then just get the 520 and if you find the power section inadequate for the rears just use the 520 as a preamp.  Most of the improvement will come from the RC, and you don’t need the processor for that as it is the same software in the AVR