I actually ended up using the car seat on the pram a lot more than I expected! Popping to the shops, in to appointments etc. also helps when they are sleeping (my son did not appreciate being moved when asleep!).
I got a cybex car seat which you can actually recline so it almost lies flat - which was an absolute life saver as they can stay in it a bit longer more comfortably. I used it loads on my pram in those first weeks/months
I used the car seat in the pram a lot with my first, I barely bothered with the bassinet. It was convenient for just clipping on from the car and off you go
I didn't use the car seat with the pram once. You're only meant to keep small babies in the car seat for a short time so for any trips out the car this always meant the baby would be in the car seat too long so I preferred to use the bassinet so they could lie flat safely.
I also got the cybex lie flat with my first and used it on the chassis most of the time unless we were going on a long walk then I used the carrycot. Definitely worth it for those quick stops at the shop on your own x
I'd say go for a separate car seat, ideally you'd use the pushchair for 3/4 years but the carseat would only be used for 12-15 months in most cases so shop around separately x
if you go to a proper car seat specialist they will be able to do you a travel system set up, we are going for the bugaboo donkey duo with the avionaut pixel pro from Little Bubbas. I used the car seat a lot with my first for lots of short trips and then bassinet for any longer trips, it definitely helps to have the option to use both on the pram depending on the length of time you’ll need baby in it
You can get adapters so it doesn’t matter which car seat you get, it will fit on your chosen pram
Definitely get the pram you want and then the car seat separately (strongly recommend car seat safety group on Facebook for safe car seats as travel systems car seats are not recommended). Most adapters are universal for prams and you’d be surprised how much you actually do use the car seat on the actual pram especially when nipping in to the shops!