Re: Need a new convertible to replace Scenera
The Evenflo Triumph 65 is sized similarly to the Marathon, although a bit wider. It has a no-rethread harness and is usually $130-$150. The roundabout 55 is exactly the same size as the marathon, but you do have to rethread the harness to adjust the height. It's usually $150-$160. The Safety 1st Complete Air might be worth a look, sized similarly to the marathon 70 - perhaps a bit narrower, perhaps a bit taller. And if you really like the marathon 70, sometimes you can find those for under $200.
Seats that are definitively smaller than the marathon are more likely to be outgrown RF before age 3, but you might look at the evenflo tribute and see if that seems like a viable option. It's way under your budget, and you do have to rethread the harness to adjust height, but it is small. There may be some smaller seats that I'm not thinking of as well... but, like I said, you'll want to check for growing room before you buy.