Need help choosing seats for upcoming air/car travel with 7yo and 2yo

Hello! I need some advice on which seats to take on our upcoming vacation. We will be flying across the country to CA and then staying there for 5 weeks. We will be renting a minivan for the duration, but I do not know what the make/model will be.

I have a very tall 10.5 year old who five steps in pretty much every vehicle we've encountered, so I don't need a seat for her. My son is a few weeks shy of 8yo, 51 lbs, and 49 in. At home, he has been in a backless SK Monterey for a short time, but I've been wondering if he should still be in a HB booster. He outgrew the HB portion of the Graco TB a while ago, if that gives you an idea of his torso height. I have two Montereys but no other dedicated boosters I can use. I am more than happy to buy something new for him to take on the trip. So, my questions regarding him would be:

1. Does the moneterey (with or without back) make a good travel seat? It seems so big and awkward. Will they let me carry it on the plane and count it as his one bag?
2. Is there another booster that travels better?
3. Do you think he should still be in a HB or is a LBB fine for him this summer (and after we get home)?

My daughter is 2.5. She hasn't had a checkup in a while, but she was in the 97th percentile for height at her 2 year (36 in) and the 50th percentile for weight (I don't know her exact weight but she weighed 25 lbs exactly one year ago). She rides RF in either a Britax G3 Advocate or a Graco MyRide. I also have a Safety First Avenue that I bought for travel when my son was younger and it still has a year before it expires. I have traveled with Britax seats before and while they install like a dream almost anywhere, they are heavy! And the MyRide is just huge. I was hoping to take the Avenue, but I've never tried to install it in a rental car, so I'm not sure how that would go. My questions regarding the 2yo:

1. Would you take the Avenue?
2. I was planning to FF her on the plane, regardless of which seat I take. If installation RF is a problem in the rental, would you FF her for the five weeks?


Is there anything else I should know about flying with kids? The last time I took the kids on a plane was 6 years ago. Any advice you can give me is much appreciated!

I would buy a Harmony LiteRider for the almost-8 year old and bring the Avenue for the 2.5 year old, FF on the plane and RF at destination.

ETA: If I couldn't get an acceptable RF install and could get a safe FF install I would FF without question.

Thanks for replying! So it sounds like you think my son is ok in a backless full-time? Or do you think we should put him back in a HBB when we get home? Is the literider much smaller than the monterey? I had a Harmony cruz (and LOVED it)--how does the literider compare to that one? For carrying a booster on the plane, do I have to tell them anything at the gate, or do I just walk in and put it in the overhead?

For the Avenue, is there anything I could put in my bag, like a towel or something, that would be helpful in getting a successful RF install in the rental?

Thanks!