Which Britax?

My LO has peed in her 4Ever a couple times, and I am done! She does well, unless we forget to have her potty before we get in the car. Our fault, really, but this last time was just too much. I already ordered a new crotch buckle once after it got soaked and retained the smell. This time, she got the crotch buckle, the lower anchor strap, the harness adjuster strap, and the plastic base. It even got to the vehicle seat. I cleaned as best I could, but I am pretty sure the stink will stick around.

I was planning on using the 4Ever until she is FF, but it won't fit in my car FF, so I was planning on getting a combination seat next year. Now, though, I want a Britax convertible so I can use a seat protector. I can't see that any of the other solutions I've seen to protect the seat will work for us. I don't really want to order a bunch of replacement parts for the 4Ever just to have another accident, either.

V is about 37", 28 lbs., and 3 years 5 months. She is small, but has a longer torso. 3t pants are a little long on her, and 4t shirts fit her nicely. In her Scenera Next (grandparent seat), she is just about to the top harness slots, but with her small head, she still has growing room. She has to RF to 30 lbs. in my state, which she should hit next year after she turns 4. She has to be harnessed to 40 pounds, which she may not reach until she is 7.

Which Britax seat will last her to booster size? I don't really like the Marathons, but am considering the Roundabout and either version of Boulevard, mostly. Thanks!

Edit: forgot to mention that the seat will be in a 2017 Corolla, probably outboard. Outboard seats have fixed, slightly forward-leaning head restraints such that the only combination seat I tried that worked was the Pioneer. (I assume Frontier/Pinnacle would fit too.) I have access to BRU to try seats.

I would probably get a boulevard for her. The roundabout is great but the top harness height is too low to make it worth it. I doubt the roundabout would get her very far forward facing if she is already in 4t shirts.
My kids fit the non ClickTight boulevard until at least age 7. My 5 year old has a ton of room left in it at 37lbs and 43”. She was similarly sized to your dd at age 3.5.

Good news! I got a June 2014 Boulevard from a trusted, very safety-conscious friend of mine! V actually has over an inch to grow over her head RF, and it is old enough to allow RF tethering! It also fits great FF. It expires the end of the month V turns 7, so hopefully it will get her to the 40 pound booster minimum in my state. My waterproof seat protector pads are on their way, and I got the cover and pads all cleaned today, which was lots of work.