Isofix aren’t necessarily safer. Just that there is more margin for error with seatbelted seats. It’s also still possible to get isofix wrong as well.
Having been through 5 car seats so far, the one I like the most is one of the cheapest. Mothercare Madrid, £50 sometimes in the sales. It’s a baby seat and works great for our 18mth old. It fits with a seat belt and faces forward or back. Great recline on it for them to sleep in too.
Our 3 year old is tall and has been in a high backed booster since before his 3rd birthday. They’re great once the child is tall/heavy enough not to use the straps as it just uses the seatbelt in the normal way, a breeze to use and relocate.
I’ve had both of them in the front seat.
Re: airbag, if the child is rear facing then turn it off. If the child is front facing, unless the seat base is above the dashboard, I’d leave the airbag on as it will protect a correctly strapped in child nearly as well as an adult.