The Port will be fine. You may get more space when fitting the car seat with seatbelt, but it might also be worth keeping the base for the convenience, if it does not take up too much room.
Zoomi and Every Stage are both quite wide, so it would be useful to look into changing these.
The every stage could be changed for an Axkid perhaps, these are good for getting 3 across and are also belt fitted. Assuming 6yo is using the Zoomi as a HBB? You could try that next to the Axkid, but honestly the side impact protection on that seat is not great so may be worth moving it to the centre seat and/or replacing it anyway. It is also much wider at the base than it is at the top. You might want something which is narrower at the base, e.g. Maxi Cosi Tanza i-size.
I have just looked up a random Octavia manual - mark 3 2017 - (you'll need to double check for your own version and year) but just as an example of what they say.
A rear facing seat (e.g. the Axkid or even the Every Stage) may be used in the front passenger seat with airbag off. (General info not from manual: This is a safer option than a FF child in the front).
They have various complicated instructions about putting a Group 2/3 (high back booster) in the front - something about lining up the seat with the side pillar. This is likely to do with distance from the airbag.
The centre rear seat allows child seats fitted using seatbelt. It says something about an extra top tether point in the middle but it also says it's country-specific - I think this is unlikely in the UK.
It says nothing against placing a support leg on the centre rear seat, which means you could potentially fit an Axkid ERF seat there - this may help you a lot.