Hello and welcome,
Britax has a good reputation with parents, and for all we know they protect children well in crashes, and they tend to install easily, and they come in pretty colours and patterns, but where Britax falls short (and I mean very short) is in rear facing legroom. If I had a penny for every family (ie: not family who posts on a car seat forum, and/or is a tech or car seat finatic, etc, but rather just an 'average' family) who told me they couldn't rear face any longer because their child was uncomfy, impossible to load into their seat, etc, I'd be a very rich woman. I've worked with multiple families who used Britax rear facing until age 2ish and then transitioned to a Diono to continue to rear face, and it's like a while new world opened up. Finally, yes, they can imagine rear facing until age 3+.
My first thought is that if you want to continue to rear face your baby you should consider a seat that offers more rear facing legroom. Unless you're a diehard rear facer like so many of us are you probably will struggle to load and unload your baby rear facing into a Boulevard. Depending on the vehicle you have it can be quite awkward -- I've seen it many times, and I sympathize. I get it, I really do.
So, is a different seat a possibility? It wouldn't even have to be a Diono. There are many other seats that provide better rear facing legroom, and don't cost a fortune.
If your next question is something like "but I bought the best seat, wouldn't a cheaper seat be less safe? ... Is a forward facing Britax better or worse than a rear facing cheaper seat?" Or anything along those lines, please know that a rear facing seat installed and used correctly is going to provide better protection for his head, neck and spinal column, at his age, without question, when compared to a forward facing seat. Translation: I'd take a rear facing $65 Cosco Scenera over a forward facing Britax for a 23 month old, without question.
To your original question, thin warm layers of clothing followed by a one-piece fleece suit is fine, and very warm. It's -14F (-25C) where I live right now, and my 26 month old is fine. Thin warm layers of clothing underneath, head covered, hands covered, and I scoop her into my arms and cover her with a blanket when we are going from car to building.
When we play outside she wears the puffiness thickest warmest snowsuit I can find... but riding in the car is completely different.
Look at LL Bean and Land's End for budget friendly one-piece fleece suits.
Hope that helps and please excuse typos. My iPad is having a tantrum lately.