Quiet mode on my DSLR is not quiet enough. How can I make it quieter?

I know that general shopping recommendations are off-topic, and I will try to be as specific as possible, so please comment/edit if this question is not specific enough for this site, or if I can clarify anything in this question.

TL;DR: Quiet mode on my Nikon D7200 is not quiet enough. How can I make it quieter, or what should I buy to make it quieter?

I am a Nikon shooter (D7200, 24-70 f/2.8, 70-200 f/2.8) and am not looking to change ecosystems. I shoot mostly for fun, but have recently been hired to do some events (plays and lectures, mostly). Generally, sound is not a problem, but a few days ago, an opera company asked me to shoot their show… during a live performance. I warned them that my equipment wasn’t silent, but I put my camera on quiet mode and they told me to do the best I could. That didn’t stop me from gritting my teeth every time I pressed the shutter button. As I make more and more connections, I foresee myself doing more events where my equipment needs to be silent, or as close to silent as possible. I love the manual control that my DSLR offers me – I always shoot full manual and am constantly using the dials to adjust shutter speed, aperture, and ISO on the fly.

I am looking for one of two things. Either,
1) A way to make my DSLR quieter without inhibiting my manual control too much (i.e.: not a sound blimp) – either DIY or a purchase recommendation, OR

2) A recommendation for the purchase of a new camera body that I can use my Nikon lenses on that will allow me to shoot quieter. Would something like a Sony A6500 and Nikon lens adapter be significantly quieter? How does the Nikon D850’s silent shooting mode (electronic shutter) stack up (note that this is a bit more expensive than I would like, but if it is the best option, I would be willing to look into it)?

Note that because I am looking at purchasing this as an investment for paid work, money is a factor, but is less important than function.