Been there, done that.
I would highly recommend a separate 2 channel system that would also connect with your Marantz AVR via its pre-outs to the ’front’ channel/speakers.
You could find a very good integrated with HT pass-through which when turned on, would power your ’front’ main 2 channel speakers, or when off, would provide the source for you music. Your Marantz would always provide power/signal to your center and rear speaker in HT mode.
Your streamer could probably be connected to both a new 2 channel integrated via the analog outputs, and also the Marantz via a digital out should you want that flexibility. Your streamer also has its own DAC, so you don’t *need* a separate DAC, but that is always and option.
In this way, all your 2 channel listening would be via the new 2 channel integrated (or separate 2 channel preamp and power amp). In HT mode, the Marantz is used for powering all but the ’front’/main speakers and 2 channel analog sources.
I tried doing similar to your setup, and used it for both 2 channel and HT, but the 2 channel SQ was always compromised. Initially I got a separate power amp to use for the fronts, that helped. A little. But the real change was when I went to a dedicated 2 channel preamp with HT pass-thru, and connecting all my 2 channel components to that, not my AVR. The sound quality increase was substantial.
I now have the ’best’ of both worlds. When I want to watch TV, or the Berlin Philharmonic in HT mode, it sounds great, and when I want to just listen to 2 channel music, that is now great too, as I got the AVR out of that chain with components designed and built specifically for that purpose.