2014 Odyssey 4 kid carpool

I drive an Odyssey to work and drop kids off at school on the way, the carpool is the most important/most common seating arrangement for the car. I have been overthinking this too much, and would like someone to just tell me what to do

Next school year I will have the following kids for the 35 minute drive to school:

everyday:
my 3.5 year old daughter, average height/weight but long torso
my 6 year old son, slightly small.
6.5 year old girl, average/slightly skinny

Plus, once or twice a week
5 year old boy, average height/weight

Currently my daughter is not in carpool, so the extra boy (will turn 5 over the summer) rides in her car seat when he's with us. I have the bigger kids in the 2nd row captains chairs in a Britax Frontier and a Graco Nautilus, and my daughter's seat is in the 3rd row outboard in a Britax Marathon from 2011. The 2nd row middle seat is removed, so a grownup walks past the frontier and through the middle "aisle" to buckle the 3rd row child and the bigger kids can unbuckle him/her. However, it's kind of a pain right now when the boy's parent has to climb over my son to get their kid buckled in -- but we deal with this because having one of the big kids in the 3rd row on the day the smaller one isnt' there wouldn't make sense, and our car doesn't allow the two captains chairs to be next to each other (could do it with one captains chair and the middle seat in theory, but not with the seats we have).

The Nautilus is not ours, it belongs to the carpool family. They have said they want to take it back next year for their other kid and give us a booster instead - and I can let them know what kind would work best in my car.
My son can do the buckles on the Frontier (harnessed) just fine, but really really wants to be in a booster seat "like all my friends". I'm sure this will be especially true when the 6 year old girl is in a booster!

I am willing to buy a couple new seats to make this work. Factors I am considering:
-the two 6 year olds would like to sit next to each other. My son can do buckles just fine, his friend still needs help
-my kids will get in first, then the 6 year old girl next, then the 5 year old boy last.
-my kids will ride in the car on the weekends (and occasional weekday afternoons/evenings), and I'd like to avoid having to crawl over large installed car seats to buckle them in the 3rd row--it's easier, for example, to get past the frontier than the nautilus right now.
-The marathon will expire toward the end of next school year. I'd like to keep my daughter harnessed as long as possible, so I've figured we'd keep her in the marathon until it expires, then move her to the frontier and buy a straight-up booster for my son (who will be almost 7). But if it makes more sense install-wise to have him in a booster all year, I'd be comfortable putting her in the Frontier (I know it converts to a booster, but if we are buying something new, I'd like to buy the cheapest category and keep the more expensive one in the youngest-kid setting, if that makes sense).
-I don't have a lot of extra time in the mornings, and do not want to do any seat-installs on a regular basis. Obviously things come in and out occasionally because of guests or friends or whatever, but I don't want an assumption to be that every Monday I swap two of the seats, or whatever.

Thoughts? I don't want to spend a ton of money on this, but I can invest in pretty-good seats, and the other families will subsidize or buy outright the seats their kids ride in (depending on price).

thanks!

How about the older two in the third row and the younger two in the second row? I am picturing this with the center second row seat in and one captain's chair out, but I am not sure about the seats you have. The second row would be the two younger kids, with your daughter in the Marathon or Frontier in the captain's chair. The boy's family could buy a seat that fits in the center seat (which other people know better than I do). Then the older two would be in the third row, with the girl in her booster behind the captain's chair. Your son can help her buckle and her parent can visually check she is in right. Then your son is behind the aisle so he can get in and out easily, whether in the booster or Frontier. I am assuming she could squeeze by your son to her seat.

I only have one kid and do not own this van, but I thought I'd share my idea. Hopefully others have more specific seat recommendations.

So you want 3 (high back?) boosters and a forward facing harnessed seat, ideally with one of those in the +1 seat in the center 2nd row?

Boosters tend not to work particularly well there, and none of your current harnessed seats work there, either.

For boosters, I would mostly get Graco Affixes.

Then I would either:

1) Work on teaching the oldest 2 to buckle themselves into boosters. Put the youngest in a captain's chair. Leave the center seat out.

2) Get a Harmony Defender, an Evenflo Sureride, or a Graco Size4Me/Fit4Me/MySize. Put it in the +1 forward facing. Keep your older kid in the Frontier as long as you want, or put it away and pull it out for long trips if you want to reharness, etc.