So, when we buy my plane ticket to Canada (he would obviously have a round-trip ticket), could I just get an open ended ticket? Or go for three months (that's how long I was there back in July), obviously aware that I may need to go back? Would I still need ties to America to prove I'd go back if I was made to go back? Or are they good at allowing people, who obviously start the sponsorship and PR processes, to extend their stay?
-Open-ended ticket probably fine.
-Don't show up with all your stuff, don't say you're completely moving permanently tomorrow. It's a visit for now.
-Tell the truth when asked about things, yep, you're married, yep, plan to do PR sponsorship, timing not yet certain, for now it's a visit.
-Yes, having ties and having to go back for stuff or whatever is a good reason to not say you're 'moving' right now.
-Then when in Canada, do the PR sponsorship, and apply to extend your visit when the time comes.
-Yes, they routinely grant these extensions when spousal sponsorship in process.
To put it bluntly, the border officers can sometimes be a bit sensitive about people showing up to move - permanently - without doing the proper paperwork. That's why it's better to make clear it's a visit for now and you plan to move later. If in the end, plans change and you don't go back during process, that's a change of plans. As long as you maintain status by extending.
[Side note: you'll see in procedures you can do a spouse-in-Canada process or family class; colloquially referred to 'inland' and 'outland.' Short form, probably do the family (outland), even if in Canada. There are no benefits of note to inland in your case, and some downsides (albeit mostly technical).]