Hello and Welcome!
That's some good use out of the Coccoros! It is a very reasonable option for a typical 4-year old, 30+ pound child to transition to a combination harness/booster like the Nautilus. It's also fine for a 6-year old to use a booster, as long as they are mature enough to remain seated properly in it. That would leave your baby in the rear-facing Fllo which is a great option.
If you want to keep the 4-year olds rear-facing, you might consider something like the Graco Extend2Fit 3-in-1. This model has generous limits and extra legroom for older rear-facing kids and also converts to a high back booster later as well. Target has a nice deal on the Stocklyn fashion right now at $165:
https://bit.ly/3dJfOSa
As for arrangement, you probably want the rear-facing toddler in the 2nd row? If the 8th seat is usually stowed, you could have the aisle in the middle or slide the passenger side seat toward the middle leaving 3rd row access on the passenger side. Since you have top tethers everywhere, I'd suggest keeping the booster rider in the other 2nd row seat. That makes it easier to put an adult there when needed.
Then, you can install the Nautili or other combo seats on each side of the 3rd row seat using a top tether.
Nautilus Snuglock LX has a $25 off instant coupon at Amazon right now if you like the Codey fashion:
https://amzn.to/3bpPDOB . That makes it around the same price as the Extend2Fit 3-in-1. The Nautilus gives you the SnugLock seatbelt installation system and a backless booster mode. The E2F3n1 gives you the extra rear-facing limits and legroom. Can't go wrong either way since you have a minivan and should have enough room to put more upright rear-facing seats in the third row if needed. Still, make sure you get a free returns policy just in case! Target and Amazon (as long as it's sold/shipped by Amazon and not a 3rd party seller) are good with free returns, as is Walmart.
My almost 4-year-old twins (31 and 33 lbs) have outgrown their Combi Coccoros. I was thinking of using this as a reason to turn them forward facing and put them both in Graco Nautilus Snugfit LX. Because of quarantine they haven't ridden in the car since March and I doubt they will again before their birthday in June. Should I buy something rear-facing instead?
My 6 year old (44 lbs) is currently in a Clek Fllo and the baby (8 months, 17 lbs) is in a Chicco Keyfit 30. I am thinking of giving the Fllo to the baby (rear facing of course) and moving the big kid to a Graco Turbobooster Grow, and rearranging all the seats while I'm at it. Too early for that?
What sort of seating arrangement works best for these seats in a 2007 Honda Odyssey? I have the 8th seat but usually keep it out. Other thoughts?