I've gone through a similar transition with my Denon X3700H, only to a 3.1 system. I think you'll find eARC is going to work better for your Netflix app than the optical audio out.
What are you using for speakers at the moment? I would advise adding a center channel.
First of all, I don't use the center channel on my audio CDs. I find it obtrusive and does not image as well as having it off. In addition my Totem Arro's L+R speakers list at $2000 CDN a pair while the centre channel was $650 CDN, so not quite in the same league.
Totally different story on Netflix. I put on the 'Formula 1' series and with the center channel present, the voices are much clearer isolated on the center channel versus down mixing into stereo. And the sonal background is much more impressive. The 5.1 sound for that series has been engineered very carefully and really suffers when you downmix to stereo. The fact that the centre channel isn't at the level of the L+R does not matter because of how the center channel is being used. I mean we're talking badly recorded people talking while walking with race cars in the background. Lots of oomph, but it's not like hearing a Mozart violin concerto.
I have gone with the philosophy of trying to present the original source sound to the Denon receiver so that I can control the sound all in one spot.
I have not been affected by the downmixing bug, and to be honest, I don't have a thorough understanding of when or how it would come into play. I've been told by some people that it might affect me, and by others that it won't. So far, it hasn't and I've done lots of testing.
I could say more about the transition. After all I wrote 2000+ words on it, (link below). But hopefully this helps a bit. Happy to discuss this further though.
https://www.audiosciencereview.com/...he-denon-x3700h-avr-and-center-channel.22932/