How should I optimize for compensation?

However I'm concerned my RSU grant will still be the same, so is my only option to switch companies or perhaps boomerang back in the US?

This is a pretty tactical question that should have a clear answer, especially if you work at Big Tech. I would be surprised if you couldn't get the answer from talking to coworkers and looking at Blind.

From my experience, the RSU grant should be adjusted as well, but I admittedly don't know any engineers who came to the US via a Big Tech internal transfer super well. I did see the opposite direction where a Meta teammate was forced to relocate from the US to Canada due to visa reasons, and their RSUs were adjusted down (they were a Staff Engineer so it was a huge pay cut).

When it comes to being in the US overall (and specifically California), the pay is definitely a lot higher than everywhere else, but you need to deal with the problems of, well, living in America. The healthcare system is a complete mess (even if you have a nice Big Tech healthcare plan), crime/violence are much higher (especially gun violence), and the overall social safety net is extremely poor. More thoughts here: "How important is location for software engineers?"

The real benefit of living in the US IMHO (and specifically Silicon Valley) is the talent quality and speed of innovation. The money's nice (and it really isn't too nice as so much of it is canceled out by sky-high cost-of-living), but the real career asset is growth. You'll be surrounded by way more smarter people, particularly in the AI space (so many of the world's leading AI companies are in SF, including OpenAI).

However I'm concerned my RSU grant will still be the same

It might be worth asking the HR department whether your company adjusts initial RSU grants if an employee transfers locations.

It's possible that this is too much of a micro-optimization to reset the RSU grant. If you are happy with your team and the company right now, your energy might be better spent doing well at work. But, if you are thinking about transferring, it's probably best to do it as soon as possible, esp. if you aren't tied to Canada and want to plan to live in the Bay Area in the future.