ERF, easy to move between cars, simple to fit?

[meconiumhappens]

Hi :)
Am looking for ERF (either up to 4 or 6 yrs is fine) carseat but it will be used in both my car and two grandparents cars. It will need to be pretty easy to move between both cars and as simple to fit as possible. The grandparents have a fiesta and a C3, my car is bigger. We all have isofix.
Any ideas would be great, thanks in advance :)

Hi, for moving about all the time I would recommend the britax two way elite. It's super light, easy and quick to install and lasts to 25 kgs. We got a set of tether straps to leave in each car as they take the longest to install.

My hubby regularly moves the triofix between his and inlaws cars as they have ds1 2 days a week. When I told someone it took him 2 mins he corrected me and said 30 seconds!
It's very compact and has isofix base which seat clicks on to. Rf to 18kg then ff to 36kg as high back booster.
We also have a klippan kiss2 which is rf only and goes to 18kg but does have 2 recline positions which triofix doesn't. Same thing though - isofix base and separate seat and pretty compact. £100 less than the triofix.
I rate both seats highly and for ease of moving would avoid a seat with tethers (I do have a TWE & Max Way so I know they're not difficult to fit but they are more time consuming).