Is your child on the tall side? If so I would avoid Britax seats as they have a shorter shell than many competitors, which translates to being outgrown sooner both forward facing and rear facing, so a lot more money than many seats for shorter use. Their seats are also reputed to have terrible leg room for RF children, which becomes an issue the older a child gets.
The radian would be a good choice, but you would need to make sure to use the angle adjuster with it or it takes a ton of room RF. The official rule from diono is that the angle adjuster is for babies 12+ months, but most techs I know are comfortable allowing people to use the angle adjuster once the baby has the head and neck strength to sit independently. If your baby isn't doing that yet, I would keep him/her in the infant bucket until they are and just leave it in the car and wait until you can use a radian.
Another option if you're willing to spend a bit more is the clek foonf, which has the tallest shell on the market (meaning longest lasting both RF and FF) but a low profile like the radian. It's rated for 6+ months only, so I'm pretty sure that while it's tall the angle is such that having it RF in a small car should be fine (I believe it was designed with that in mind) although I haven't seen one in person or in use so I can't make a guarantee on that front.