lol if any of my few family members want to drive my kids then they can buy their own (my aunt has been looking for one that will be able to fit from 0-4y so she will be able to drive any of my 3). As for taxis we didn’t use one - they either sat on our lap or in their pram seats strapped in. Unfortunately I think most car seats are just one of those bulky/heavy things. @Csilla suggestion makes the most sense but it seems like you’ll still be set back on cost
We have liked the graco 4 ever car seat. It grows with the child. And we have 3 with 2 kids. My mom bought one and then when we moved away she gave me hers. So we have 2 in normal family car and one in my car I prefer to drive
I know you said you don’t like the doona but buying the base for others people’s cars was easiest and most cost effective for us
I have a Graco with 2 bases and a baby trend travel system where the car seat goes on top of the stroller. Works well for us
I would just get a car seat that sells the base separately and buy several bases for the one car seat
Maybe a nuna pipa RX with base and buy multiple car bases, so it can just click into place in each car? It can be attached to a stroller base too.