I agree with doing your research now, before you're in a critical point. My daughter, too, has had an unpredictable growth pattern and is prone to spurts. She was in the 5th percentile for weight at 6 months. She started catching up after that and was about the 15th or 20th percentile at her one year checkup. She was still under 20 lbs at that point, though I don't recall her exact weight. At her 2 year checkup, she was in the 90th percentile for weight (95th for height) and weighed in at almost 31 lbs. At 25 months, she still weighed in at the same almost-31 (30 lbs 12 oz or so). I weighed her a couple of days ago at 26 months, and she had gained 3 lbs in one month; she's now 33 lbs 14 oz. I weighed her because she'd been acting like a bottomless pit who can't get enough food the last several days and I was suspecting a growth spurt. She seems to be in the middle of one and who knows where she'll stop? I bought her Radian before she was born because I needed something for kids I was babysitting anyway; at her 1 year checkup when she was still under 20 lbs but average height, I was starting to wish I'd gone with the True Fit, at least in the Suburban where I have extra space. Boy am I glad now that I didn't do that. I'd be getting pretty nervous since I'm nowhere near ready to turn her forward facing and she only has a pound or so to go in a TF.
I believe your only two options for RF past 40 lbs will be the Diono Radian R120 or RXT (RF to 45), or, if you can afford it, the Clek Foonf (RF to 50). The Foonf isn't shipping yet but should be very very soon (within the next few weeks in the US, and I believe they were planning to launch in the US and Canada at the same time; Clek is a Canadian company). You should have time to wait for it if you want. In the US we also have the Peg Perego convertible that will rear-face to 45, but I don't know whether that's available in Canada.
The Radian can use an "Angle Adjuster" to put it more upright. It's a $10 optional accessory made by Diono. It should solve any "fit" problems you're likely to have in your car as far as front-to-back space. The Foonf also can install quite upright if you forego the optional anti-rebound bar... I believe... of course rules may be different in Canada and everything is subject to change once the final product is actually on store shelves, so be sure to double-check that info when it's actually available. I've tried a Foonf prototype in my Honda Civic and I wasn't super happy with how it fit but it was doable. I wasn't able to install it in the center, I might have liked it better there, but I had my toddler with me (then 1.5 years old) and I didn't want to move her car seat out of the center and then reinstall it. And it was raining. Of course.