I'm a geezer and have made many digital upgrades over the years. In my experience you are not going to achieve a "Wow" improvement by upgrading your digital gear or even your preamp. Expectation bias has been mentioned and I appreciate your thoughtful approach. If you spend 3 grand on a new streamer or other component you can certainly convince yourself that it sounds much better.
The best ways to get a big difference in sound are not available in your situation. You can't move the speakers, can't rearrange the room, can't install sound treatment, and you already have an excellent amp and very good speakers. From this point forward any money you spend will be directed at convenience, features, pride of ownership. or another justification. But a big leap in sound quality is going to be elusive.
I do have one bit of advice regarding gear. I suggest that you check out the reviews of the Eversolo A8 mentioned in a previous post. This looks to be an excellent product and the features and ease of use may be worth the money in your situation. The A8 is going to be my next gear purchase even though I don't need the built-in DAC (I have a Berkeley Alpha Reference Series II MQA & Alpha USB) but I'm streaming now through a laptop and the A8 looks to be more convenient and elegant. BTW, the SQ of my CD playback (Jay's Audio CD3 Mk III transport through the Berkeley DAC) is virtually identical to streaming the same song/version through my Asus gaming laptop. I really doubt that spending 5 figures on a streamer is going to make Qobuz sound better than my CD rig.
Another suggestion would be to hook up your Oppo as a transport to your Gustard DAC and see if you can hear any improvement. My guess is that they will be very similar but you have a nice DAC so you might as well use it for CDs.
With the constraints you have you are fortunate that your rig sounds very good. I think it's time to chill and enjoy the music.