This isn't about taxes, but I suggest you talk to your brokerage firms to make sure you can still invest the way you want to, or at least learn the rules in advance, so you can take action before moving your residency to Canada.
I am a US citizen living in Canada. Before my move in 2015, I had all my investment accounts (After tax, rollover IRA, and Roth IRA) at Vanguard. Vanguard has very tight rules for Canadian residents (not with UK or MX residents from what I can see. Check out Financial Wisdom forum), and Vanguard reps (I called them multiple times talking to multiple people to confirm) said all my accounts, including my rollover IRA and Roth IRA will be frozen, except for selling shares. Fidelity said the after-tax accounts will get frozen (except for selling shares), but I can still trade within my Roth and rollover IRA as long as I already have such accounts with them BEFORE the move. (BTW, they will not do EFT transfer of your money to a Canadian bank unless you already had a Canadian bank defined before your move to Canada, which I found out after my move.... ) So before my move to Canada, I had to move all my money from Vanguard to Fidelity. I have done partial rollovers of my 401K to my Fidelity rollover IRA since my move without any issues. I imagine things may be different if you keep an address in the US, but I didn't want to go that route.
As for taxes, I heard 85% of social security is taxed in Canada, but it sounds like you are going back to the US before Age 65, so as long as you take SS at full retirement age, I don't see that being an issue for you. This is only my first full year in Canada tax wise, so I don't know for sure, but running tax estimators online, it looks like my tax burden in Canada will be higher. I didn't move here for the healthcare, but just as a comparison, I do think the extra money I pay for taxes is probably less than what I might have ended up paying for for healthcare in the US.