I don't know if this was the program we used, but you can get a LOT of free stuff from Microsoft as a startup. https://azure.microsoft.com/en-us/overview/startups/
>Between Python and .NET ecosystems, I'm more familiar with the former and, under equal conditions, would choose it over .NET.
I think one thing you need to do when creating a startup is, go with what you know (as you said). You want to decrease the risk that you're product is going to be blocked by encountering an edge case in a framework/language you don't know and you get stuck there for a long time. I am way more familiar with .NET than Python, but I use Python in my spare time and on side projects to build up those muscles.
If you're doing a startup and you want to go .NET, I don't think there's anything holding you back anymore. It's open source now, runs on Linux, and you don't have to pay a dime to develop with it. Microsoft will get you in that, it's WAY easier to pay a little bit to get Visual Studio Enterprise and to then hook that into Azure and all of a sudden you're paying them a ton of money. Yes, that's their master plan. But, no one is making you do that.
Where to stay away from .NET core: Lambdas. The startup time for Lambdas just isn't where it is for other languages, when I last looked at this. If someone has newer data to prove me wrong I'll freaking LOVE YOU FOREVER!
Anyway, depending on what and how you're building your startup, .NET maybe a great pick for you depending on your background. I wouldn't force myself to pick up a new ecosystem and do a startup if I was focused on delivering a product I wanted to deliver and not learning something new and cool.