Mini Countryman car seat

Hello! We recently traded our Mini Cooper for a bigger Rav 4. My husband still drives our Mini Countryman though. Although an suv, is still doesn't have a ton of room. We want a convertible car seat for that car and the infant seat will be in my car so I can take it out and use it as a carrier. We got a Britax Marathon click tight and I just installed it. In order for me to use this, I need so slide my seat way forward and sit very upright. It isn't the most comfortable position. Also we got two of the convertible chairs for when baby outgrows the infant seat. Anyhow, I was wondering if there are any good, smaller convertible seats that anyone might know of, recommend that might fit in my countryman. If you have any first hand experience that'd be awesome! I know convertibles tend to be a bit larger so I'm not sure anything will work but I'm hoping!