Car appreciation... the aesthetics, the engineering, etc

I need to replace 2 tyres on the car as the tread is very low (barely above MOT limits), and another 1 has a little more so it makes sense to do that too.

A few questions:

1. Is it worth just doing all four at the same time from a driving/safety perspective? The 4th has a couple of mm more than the 3rd, but it's not like it's significantly newer.
   
2. I'm on Pirelli P Zero that are made for the Volvo, should I stick with the Volvo specific blend?
   
3. Should I even stick with Pirelli? Also thinking of Michelin Pilot Sport if I end up replacing all four and can think of another brand.
   
4. Black circles quotes a per tyre price, but no discount if you do more than 2 at a time... Do places do discounts for multiple tyres or should I just expect price x 4?
   
   

For reference this is what I run today: https://www.blackcircles.com/catalogue/pirelli/p-zero/245/35/R20/W/95/f?tyre=39897313

I've got about 22k miles on them, they're the set that originally came with the car, although two experienced punctures in the first year in South London.