Just commit and sell the car, our driving was about double that and we do not miss it a bit now. Still drive about 1000 km/year in rentals, but we are trying to minimize that.. It's especially nice as a family to primarly transport yourself in other ways were you get to spend time together interacting freely instead of being stuck in a car.
Economically it's more or less the same, but a better quality of life.
We've thought about it. One of the main barriers is honestly the combo of a) out of town family and b) young children. Installing carseats and boosters in a rental is a giant pain, and the train/coach schedules aren't dense enough to permit a reliable day trip to a location that's a 60-90 minute drive away.
We do have another family in our circle are are carfree and they make it work with a combination of rentals and every now and then they borrow our car in exchange for babysitting and other favours.
Depends on the seats. We have seats that you just put in. They are held in place by the seatbelt. So there's a bit of extra carrying the seats, that's all. We bought them because they were top scoring in a crash safety test.
The real gap is with short-term car shares where you want to walk to the car, do your errand, and then walk home. Most carseats aren't great for being carried on foot or taken on public transit. So that leaves you with only doing rentals when the time period is long enough to make it "worth it" to bring the car home on either end of the trip for seats in/out.
Economically it's more or less the same, but a better quality of life.