What kinds of sources (disc players, turntable, etc) ? No 4k involved at all nor Atmos?
Back speakers come after you add side speakers (5.1 uses surrounds to the sides, 7.1 add rear surrounds). The third same speaker for center is often better than the generally compromised (for fitting under a tv) center speakers, too.
The Monolith 7ch is a power amp, not a pre-amp, but would require any avr you choose to have a full set of pre-outs. Passive bi-amping is a waste of wire IMO, much more about marketing for both avrs and speakers. The age of the avr has to do with features more than anything else, altho pre-outs used to be more prevalent in receivers at one point.
ps Mid-fi is a bullshit phrase.....it is hi-fi but doesn't cost enough for those who believe higher prices mean higher fidelity.