It's all about preference. There is no "right" answer, here. Just lots of great suggestions
Particularly the quality over quantity remark. So true. Better to have a great stereo set up than a lesser 5.1/7.1 setup.
Random note: I have a setup where I can use any of my 9 speakers to create 5.1.4, 7.1.2, etc... Dolby Atmos, if set up correctly, is absolutely the leap forward in sound that people say is. It's just unbelievable. And to these ears, in my space, 5.1.4 is the best experience.
That said, I don't see games having Atmos any time soon LOL. I mean, who knows?
Point is, if you plan to watch movies as well with this system, Atmos is unreal, if not a bit limited in content.
I would suggest, and it's just an opinion, that you buy a great 5.1 system if money allows, and enjoy it. If you feel that it lacks after a time, add the other 2 channels. And if after that, you still want more, height channels! Grow into it
You can always use the $ you save buying 5 speakers vs. 7 to upgrade the receiver to an Atmos one now, and be ready for growth...
Just a thought?