The ordering of 2 car seats to the destination makes a lot of sense if your friend can meet you at the rental car place.
The 4 yo is old enough to pull her own little suitcase and wear a small backpack. This small suitcase can have a change of clothes for everyone and a few other items. My 4 yo regularly pulled her own suitcase. No need for a Trunki.
The two yo can also walk with a small backpack.
Fill the two backpacks with toys/snacks/inflight entertainment for each child.

Then a car seat stroller for the infant, a cross-body diaper bag (I love diaper dudes messenger style one) and a small backpack for you. You put on the diaper bag first and then the backpack. Been there, done that.

Check bags - have someone help at drop-off. At arrival, if you can limit yourself to one large rollerbag, you'll be ok.

Fly direct if you can. If not, give yourself a longer than you think you need connection for snacks, potty time, diaper changes, meltdowns, etc. Expect to walk very slowly. Don't be afraid to bribe your kids for good behaviour.