tbh, it is v v likely that once the baby is 6mo you will be fed-up to the back teeth with your super-duper-whizzy-all-in-one pram, and want something smaller and lightweight.
If you walk a lot, then the big ones are lovely. If you drive places and walk a little when there, then the big ones are a massive pain in the arse.
For ds2 I had an enormous rear-facing 3-wheeler. I LOVED that pushchair. But I almost never put it in the car - it lived in the garage (unfolded) and was used every day when taking ds1 to school.
I also had a small lightweight rear-facer (from ds1, that I got after I realised the travel system was a nightmare) that lived, folded, in the boot of the car and was used on the vast majority of trips out.
The only times the big one went in the car was if we were going out specifically for a walk, taking dog etc, rather than going shopping.
No idea about legality or safety issues but with my massive graco I used to put the chassis in the boot and the pram part in the front footwell/seat. Would that work? I have to disagree with wanting a smaller buggy - I still love using my pram as a pushchair as it's rear facing. The lightweight buggy is handy for travelling and taking in the car but is no substitute if you're walking far with it or very regularly.