I would avoid the two language problem so option B is out of the way. If you choose option A, then you run into a risk of having to optimize some part of the code in C/C++. So you end up with the two language problem again. If you care about performance, then option C is not a bad place to be. Learning Julia is not a big deal. It would not take a seasoned developer too much time to get up to speed with it. What I found is that the Julia community is so vibrant that you can get a lot of help to move along quickly. My two cents.
Thank you for sharing your helpful insights. I will continue learning Julia as well as exploring and comparing options (in addition to Python and Julia, I'm also considering Node.js and C#, though to a lesser degree). I have certainly noticed the vibrancy of the Julia community, however, to me, as a startup founder, the problem of talent availability still represents a significant issue - community help can get us only so far. Most Julia developers are academia/science-affiliated, with a smaller number of people employed in the industry, mostly in sectors like finance and energy - so, a very limited talent pool makes it quite challenging to build a very good engineering team, at least, in the near term).
