I've ridden the Canadian once a year for the last 5 years and am pretty familiar with it. I vastly prefer the off season to the 26 car monster it is in the summer.
1. Whether or not you get a roomette, aka "Cabin for 1" (which I prefer, I like the door), or a berth, minimize the amount of carry on luggage you bring. The amount of luggage storage in either is minimal. There is no common luggage rack like on a Superliner or a big cubby like on a Viewliner. There is a small cubby in the roomette. I carry a small duffel (which is usually in service as my gym bag) and a backpack and check my big bag. I almost never check the big bag on Amtrak BTW. One thing to bear in mind is that there is an outlet in the roomette, but there are no outlets in the section.
2. In the off season, there is no restriction to access to the Park car. However, note that the first 2 or 3 rows in the dome will be marked as reserved for Prestige. I almost always respect that, although at night when I like riding in the "railfan seat" and watch the signal aspects change, I will park myself in that seat at night, but there also is almost always no one else up there then.
3. There will certainly be snow in the mountains, probably elsewhere. It will also be COLD. Prepare for sub-zero (Fahrenheit) temps if you want to detrain at station stops, particularly in the prairies. And it is often windy in the prairies, too.
4. Since the recent schedule change, adding 13 hours eastbound, the performance has been much better into Toronto. I have been following it, since I was deciding whether or not to allow a 3 day pad in Toronto to allow for a 48 hour late train, which has not been uncommon over the last year. In many cases, the train has been as much as 7+ hours late at some point in its journey, and still arrived into Toronto around noon, 2 hours early. It has been arriving around noon more often than not, and I have yet to see one that is actually late since the new schedule went into effect. With that said, allow for an overnight layover in Toronto before onward travel.
5. They pretty much do not run non-Prestige Chateaus in the off-season. 212 is the last Manor in the minimum consist. Note that they may add cars based on demand and they may add cars after you reserve (that has happened to me), so you may want to check back. Also, when you make your reservation, the way their system works is you will automatically be assigned a room, and they can then change it after its been assigned. You can do it all on one call, though.
Do sign up for Via Preference before you book, as one trip between Vancouver and Toronto in a private accommodation will be enough to up to the "Privilege" elite level (CAD $999 spend). That will entitle you to a 50% off coupon (off the full undiscounted summer fare, but still good) that you can use on any train that has "discount" inventory available. It is a deeper discount than even off-season discount.
Finally, get your plans finalized and reserved soon. For some reason, the "discount" inventory is selling out much earlier and more completely than it has in the past. I got hit by this on my upcoming trip in November. I waited to make my reservation until last week because I wanted to track the performance of the train on the new schedule to determine if I wanted to keep a 3 day buffer in Toronto. When I decided (I didn't and am only laying over overnight), there was no discount inventory available on my preferred trip and would have had to spend CAD $2700, whereas the same trip would only have been about CAD $1450 had I been able to use my 50% off coupon. I checked all departures from mid-October when the Park Car restriction comes off through the end of the year, and there were NO departures with discount inventory in a Cabin for 1. I ended up pushing my trip by a week, since the next week the undiscounted fares went from "shoulder" season (CAD $2700) to "off season" (CAD $1800). I had thought the end of the trip was set, I just didn't know whether I'd depart on the Friday or the Tuesday train because I was debating the length of the Toronto layover, so I had already booked return flights. Luckily, they were on frequent flyer miles and I was able to move my return flights back a week without penalty.