That one is difficult. You have a good HDMI 2.0 receiver now and an older 1080p projector. Basically the same boat I'm in. My pj is much older though since I got it in 2011, but still.
I vote projector. It will be a HUGE upgrade over what you have now. JVC will always be my 1st choice.
@AcuDefTechGuy can help you out all you need there and on the receiver end. My plan is PJ first, new receiver if I get something that can actually use HDMI 2.1.
Since you can go up to 9.x now, you can always do a 5.1.4 setup and not use the back speakers all the time. Would give you some room to experiment if you really need them or not. Then, when everything settles on the newer receivers and the fun that HDMI 2.1 has become, you'll be ready for the new receiver. You've got a solid receiver that isn't out of date, so you aren't missing anything but the extra channels.
EDIT: I looked a bit further into your receiver and it looks like one of the earlier models that got an Atmos update. Doesn't mention DTS:X. It also looks like it is only capable of 7.2.2 (which is weird) as far as the pre-outs on the back are labeled. I might have to change my vote to receiver first. This one is tuff.