Congrats on the new arrival!
I've got 2 kids, 4 and nearly 2. I've had my 2018 saloon from new. No problems with the boot or access for 2 kids - including for the 'travel system' when my son was a newborn. It helps that my wife is small tho as the seat in front will need to be a reasonable way forward to accommodate.
Would defo give another vote for the seat protectors as above.
Neither of my kids ended up in pushchairs for long, the baby carriers were much easier and better accepted for my two and much less bulky to transport (up to 12-18 months). That said a maclaren or equivalent pushchair is small enough not to worry.
I got both of my kids into a recaro rotating base seat as soon as I could. I think it's the hero. 1. It's isofix and the chair rotates on the base to allow forward or backward facing and you rotate it 90deg to face the door for getting the kids in and out. It's well recommended - but irritatingly rattles when empty.
In summary - space is fine, it's no estate, but definatly manigable.