Agreed, have to get an introduction to html/css/javascript before learning anything else. Maybe hold off on react as a beginner till you know the basics. .NET is ok, but again I’d wait to learn it till after getting a foundation in html/js.
Regarding corporate/startup and age. Maybe I’m a pragmatist, but what matters is can you ship and work well with others. The challenge between corporate/startup will come down to your first job. You will need a support-system at the company. I’d say most corporate companies unless they have a system in place, you will struggle. And I’ve heard terrible stories from mentees that got their first job and were fired 3 months later once the company realized they couldn’t support the individual. The upside to startups (which depending on how you define startup) they can be smaller and there might be senior engineer or two looking to mentor a new developer. So the strategy is to find an organization that has mentoring in place and it could be a corp/startup just depends.