I think you have a good plan moving forward. I agree with setting the monitors to small, and LFE only. LFE+mains makes a muddy mess, and while it CAN work, and some people do like it better, it takes mains that are much more capable with bass than the 6’s.
The reason for “small” has only to do with bass extension, and not the physical size. I wish they’d stop using that naming system.
Selling the Q’s to get monitor surrounds is a great choice too.
I also think I’d replace the sub first, and AVR later. Unless you notice it struggling, which you probably won’t, if listening at same volumes. If you go mental with volume knob though... Btw, they’re always works in progress lol.
And congratulations. I think the 6’s are a good choice, and you’ll love them.