1. What are my best options for staying in the US?
2. What other options do I have for staying in the US?
3. For an H1B visa, is there any downside to applying through multiple employers for different positions at different companies?
4. Do I have a shot for the O-1 or EB-1 visa?
5. Is it worth exploring options in Canada? If so, what is recommended for my situation?
https://www.uscis.gov/working-in-the-united-states/students-...
Word of caution. I would not mention the startup in any immigration context. I'm not a lawyer but I worry that your F1 OPT is not a blanket license to work anywhere or for a second entity even if it is related to your field.
Word of advice, get an immigration lawyer, particularly if you want to do startup stuff.
My gut tells me no shot at an O-1 and unless your are well published or worked on multi-million dollar grants during your masters little shot at an EB-1 visa. Again, your immigration lawyer is the real authority on how best to proceed though.
Canada is an option to be sure, I would again recommend a Canadian immigration lawyer but I have been told it is easier than the US system:
https://www.canada.ca/en/immigration-refugees-citizenship/se...