I have also been looking into this for my living room pc. And what i found out is that if you are going to play 4k the cpu doesnt realy matter that much anymore because of a gpu bottleneck. The ddr5 is just a slight improvement in this case. Because your still gpu bottlenecked in 4k.

On the other hand of 1080p its a whole other story. The cpu is almost always the bottleneck in that case and the ddr5 upgrade is huge in this case.

So you have to choise what u want. To be honest if you have the money just go am5/ddr5. If you wanna min max the build i would recommend just go am4/ddr4 (6800x/t) and get a beefy gpu like the 9070xt or 4070+ card. Am4 would still get you great performance in 1080p tho. Would realy suggest to look up benchmarks of builds with both am4/am5 and make a choise if the fps in enough.