Contender Slim

This is what I would recommend or the Graco Contender. Both are lightweight compared to traditional carseats. I've never used it in an Uber, but we have the Graco Contender as our primary travel carseat which we pair with a backpack bag.

My daughter HATED the car seat and we later learned she was carsick. Try an L-shaped car seat like the graco contender or the clek foonf. We tried the contender first which was an improvement, but eventually dished out the $$ for the clek because it has the highest profile so she can see out the window. She’s not 100% but much happier. I really empathize with you - it was so so so bad and I’m actually traumatized from my mat leave and having her scream in the car nonstop. And later (especially with solids) where she would throw up for any ride over 10-15 mins. Really made me not want to drive anywhere!

We used a flyaway bed on a red eye to Europe a few times with my daughter between 15 months to 2.5 and it was amazing. Now that she’s 3 I don’t see us using it again because she’s too long and prefers to stretch out in her seat and across my lap instead to sleep. We’ve also used a Grace contender car seat in multiple European countries and never had an issue. I prefer to have my own car seat that I trust vs renting one. We also use a CARES harness during takeoff and landing for added safety since we aren’t using the car seat on the plane. 

Is this something you'd buy here and bring back and forth with you on trips, or would it stay with someone in Canada while you're away? If it will stay in Canada, bulkier picks like Graco Extend2Fit or 4Ever are classic value-for-money workhorses with high rear-facing limits. The 4Ever can be converted all the way to backless booster mode, the E2F only goes to front-facing harnessed mode. If you'll be using it in lots of vehicles and need an easy install I hear Britax seats with ClickTight are hard to beat, though I don't have experience using them. If you'll be taking it on planes you likely will be better off with one of the more lightweight options like Cosco Scenera, Evenflo Sonus, or Graco Contender. Of those I think the Contender has highest limits, if you want to use it into preschool years. We travelled with the Contender and a 1.5 year old and found it easy to use; it now lives in Grandma's car and has served us well over 4 years.

The scenera is usually outgrown at 2 rearfacing so it wouldn't work long term. You could do the evenflo sonus but it's big front to back so might not work if it's a small car. I personally would just get a Graco extend 2 fit. The cheaper seats mostly all need to be rethreaded to change harness height which could complicated for them if they're switching between grandkids. Edited to add, I think the graco contender is slightly cheaper you could try that 

Unfortunately, unless it’s a seat with a closed belt path, like the graco contender, basically all seats have this issue with the buckle ending up behind the back on a plane.

We use this one too! We used it on the plane until she was 3 and then started checking it. It weighs about 15lbs, so heavier than the Cosco but more plush like a traditional convertible seat. It’s also the one we use when family is transporting our daughter.