Be careful not to conflate isofix and fixed Base as they arent the same thing.
Isofix just refers to the fitting method - seatbelt or isofix, which are little clips installed in the back seats of most cars. Isofix removes any room for user error but a proper seatbelt fit is just as safe.
Base or not means whether you have to fit the car seat directly into the car by seatbelt or Isofix, or whether you fit a fixed base unit into the car and then just click the seat onto the base.
If you want a fixed base, which is what most posters seem to be talking about, then it will be the base itself which is fitted with seatbelt or Isofix. Therfore you don't need to fit it every time because you're not taking the base out of the car every time.
Check if your car seat says non Isofix whether it means there is no Base at all or it just means there is only a seatbelt fitted Base. I think all infant car seats can be directly fitted with a seatbelt anyway, whether they are compatible with a base or not, so this may also be confusing.
I size is something completely different (except that all I size seats are isofiz) and it's not true that you won't be able to use non I size seats next year. Any seat you buy from a proper retailer you can legally use until it's reached the end of its life whether the rules for new seats sold change or not. Plus I don't think there even is any I size rule change planned next year.
Then of course there is a whole other question of whether you want the seat which goes onto the pram or whether you want to get a seat which is designed to stay in the car, perhaps usable for longer.