Too much processing?

Not at all. The units talk to each other over HDMI. If one unit converts to 4K the other units recognize it as 4K and do nothing. 

The real answer is to test all combinations and see for yourself. I did so and so:

I now run everything into my Sony LED and let it do the processing. My OPPO was not BD   ( 102? ) so I bought the good Panasonic.  The Anthem now only does the audio.  Processing engine in the Sony is very good.  So reduced the number of cables and processors it goes through.  'Back in the day" before 4K BD and I had a Visio LCD, the OPPO was a better processor.  Shame they switched to making cheap cell phones. 

Big issues with HDMI cables. Good ones and bad ones. No "magic" ones. Shorter the better.  Belden, Belkin, BlueJeans are trustworthy. Others may be OK, but I had issues and when I cur the cables open found unterminated shields. 

Two thumbs up for the calibration.  Professional best, but there are downloadable files that can take you through " do you see a difference" split screens that will do probably 95%.  I found them to be enough to get realistic flesh tones with differences in ambient light.  I was never a Sony fan, but sure do like the TV. 

Sony has a nice $499 Blu-ray player , most modern players come with upscaling 

if you have the Latest OLED that is the best picture and technologies 

LG, and Sony Bravia best OLED 📺 pictures and also have a direct hdmi out from player to tv , hdmi should ge from player , you want new ,ups calling technologies much better ,the $499 Sony is a great buy  and use a high quality hdmi cable 

I started cheap and currently $200 for audio and picture hdmi cables Wireworld I am using , but AQ is also good sharper images ,better color saturation .

if you truly want the best picture I had my tv calibrated since my TV is a very good OLED well worth it . A good tv calibration cost around $3-400 takes over 4 hours and makes a solid improvement in picture quality.

HDMI Main goes to your video, HDMI audio to your Marantz.  IIRC, the Oppo remote will control both.

So the OPPO has two HDMI outputs. It’s a little weird but only one HDMI output passes a DSD signal. I guess I could run two HDMI cables, one to the Marantz for “music” and the other to the TV for “video” (I know both will pass A and V). I guess my original question was just more about whether it’s good (or bad) to have the video signal “processed” 3x vs 2x…

Why?  Can’t you just run another digital out or analog out from the Oppo direct into the Marantz for audio?

I guess I could just try the HDMI direct to TV to see if there’s even a difference. The issue then is the audio portion - like if I listen to music from the OPPO - has to go to the TV first, then the preamp.

Try going direct into the TV and if it’s better you can just get a simple universal remote to avoid remote juggling.  Here’s one for about 10 bucks that seems to work well…

https://www.amazon.com/GE-Universal-Panasonic-Streaming-4-Device/dp/B088Y2JFHB/ref=pd_bxgy_sccl_2/143-2852872-6823163?pd_rd_w=Rwj87&content-id=amzn1.sym.21b577c4-6435-4581-8b53-49da41e27328&pf_rd_p=21b577c4-6435-4581-8b53-49da41e27328&pf_rd_r=JD4R1WFD3XM7RBDQAYYG&pd_rd_wg=q4oPM&pd_rd_r=e275c599-1c49-4c57-a443-12abddfc7bda&pd_rd_i=B088Y2JFHB&psc=1

Just a thought FWIW.