I'm also considering relocating to where the startup is based in the US in 6 months - 1 year.
However, Canada has a departure tax, and I'm wondering if I would need to pay capital gains tax on the FMV of the equity that was granted to me while I was based in Canada - and what would be considered the FMV of the equity?
You can start here: https://www.canada.ca/en/revenue-agency/services/tax/interna...
There might be differences based on your visa/residency status (many US-ians get a NAFTA visa at the border upon entry with a job offer). If some company is offering, they have some understanding of this stuff I would think.