Hi,
If you only need 5.1 then absolutely a receiver to go with.

But I can't understand an Atmos receiver that only allows two heights. The whole point of Atmos is to place the listener in a three dimensional bubble of sound where the sound objects can move freely from front to back, left to right, up and down or any combination of those. With only two heights the continuity is broken and you are left with an incomplete and muddled soundstage. It cannot work as it should.
Popping in a pair of pre outs and building in the ability to process two extra channels would seem to be a pre requisite of any receiver that wants to call itself an Atmos receiver. Having said that I know Dolby say that a 5.(1).2 system is the minimum Atmos system but I feel that is said purely for marketing reasons and not because it will actually perform as an Atmos system should.
I do have a 9 channel receiver , the Marantz 7010 and, with the addition of an external 2 channel amp, run a 7.2.4 system in a dedicated cinema room and was looking at this amp for a second system in the lounge but, as you say, I will have to look elsewhere.