Help! Receiver or preamp? Im overwhelmed...

Marantz is the best sound I’ve ever used.I bought them on eBay a year ago and they are still in perfect condition.

Seems like you like the Rotel sound. With music your main priority, Why spend the money on a video upgrade which may introduce a possible audio downgrade? You can get a 4K 4x1 HDMI switcher for about $300. It will have a 5.1 digital output to connect to the Rotel. Use rest of budget for bi wire or something.
You might find it interesting to disconnect your rears for a bit and use those channels to see how good the studio 100 would sound bi amped 

Come to think of it, you could bi amp the 100s and power rears with the Rotel 

This may not suit your purposes, but I don’t use the video portion of my preamp for purposes of convenience. All HDMI sources are routed directly to my display and I use the Toslink output of the display to forward the sound into my 2.0 setup. You could also use the S/PDIF outs from each video source to the Digital inputs of your Rotel.
I actually found that audio over HDMI sounds a little edgy and fatiguing. When I first heard audio via HDMI, I called up the local HiFi shop and asked their resident audiophiles; they told me they preferred S/PDIF to HDMI for the same reasons.
Since you like the sound from your Rotel and if HDMI switching is necessary, I’d suggest staying with Rotel for a replacement, preferably a preamp, depending on what you are willing to pay. As always, I recommend used gear. These things depreciate faster than a new car, especially when newer features & formats are added.

burndonesinkchair , yes same train of thought as mine. Other ways to get HDMI switching. To be clear to the OP, in my experience using optical out of tv would downmix the audio to 2.0 PCM. TV streaming may still be 5.1, but not external sources. Using an outboard HDMI switch would preserve the 5.1