I've seen posts like this in the past have often wondered to myself what the solution could be. There is a kickstarter solution similar to a helmet with everything cut away like you mentioned.

It would be nice to be able to use dictation out in public spaces such as cafés or even better at the office. After reading your post I got curious. I did an experiment where I moved the microphone right into my mouth and I tried to dictate as quietly as possible. I imagined I was in a café not wanting to disturb other people. While Dragon had an issue deciphering my speech when I dropped into a whisper, it actually did quite well when I spoke quietly. I imagine I could train myself to do this fairly well. But this experiment did not account for the ambient noise in a café or other public setting. I was speaking much softer than I feel like I would necessarily need to in a café. It has become much more common for people to speak on their phones, often while on speakerphone, in cafés,
so I imagine  one could speak a little louder. I would encourage you to try it.

Regarding the STENOMASK... I imagine that would draw some attention while at the café. Additionally as you mentioned they are expensive (though amazon has them for $229 and I thought they were closer to $300). In trying to think of an alternative I'm wondering if a Beltbox would work. Singers/Actors use them to practice singing/dialogue without disturbing others. They dampen the sound of of one's voice. I believe you can even get them with a strap see you do not have to hold it to your face. What I imagine is taking a small microphone and putting it inside of the belt box. This would dampen both your voice and the ambient noise.
However, I think they would also draw some attention.

 https://www.amazon.com/BELTBOX-Dampener-Singers-Performers-Portable/dp/B01FRK4MLA/

Another idea is to use Earpads Replacements. These could be had for less than the belt box. I imagine putting a microphone inside and cuping it in my hand and holding it up to my mouth. I could see needing to add additional material inside to provide more dampening to the voice.
https://www.amazon.com/s?k=earpad+replacement&ref=nb_sb_noss_1

Just some thoughts.