I think it's worth having at least two. I'd probably have a "main" seat in your car, since you seem to do the most driving, and then something easy to swap might work for the other three?
If you have a car seat that came with your Nuna Mixx (that's the pushchair model name) it's probably one of the Nuna infant carriers. Do you already have a base in all four cars, or have you been seatbelt fitting in the other three cars, or swapping the base as well? One of the Nuna seats (Pipa Lite) can't be fitted using seatbelt, so I assume it's probably Arra Next or Pipa Next.
There is a toddler seat which fits onto that base - the Todl Next. If you already have multiple bases, this is a good case where it would be worth getting the toddler seat that's compatible. If you don't have extra bases, then it's probably not economically worth it, as it can only be fitted with the base, not seatbelt.
FWIW you can use an infant seat until the baby's head pokes out of the top, which might be longer than you think. If the strap is making red marks near her neck, check the strap covers are on and in the right position (if they slide around, they are usually to prevent these kinds of marks) and if you can't move them, try pulling her clothing through to make sure it's between the strap and her skin. Because the straps are relatively tough due to their safety requirement they can rub on skin especially if DC have quite sensitive or easily marked skin. Sometimes it's harmless like a crease mark from a pillowcase, but if it seems sore, then it's fine to put something between them to make it softer. Just don't add any third-party padded items to the straps. (This always happens to my DC as well, and especially so in summer clothing).
Halfords is not actually a terrible idea in terms of them having quite a few cheaper options, although I agree their training of their staff is rarely up to much, so I wouldn't take their advice too seriously (or at least double check it later). But if you want to try seats out and see how easy they are to put in and out of the car, how your DC fits in them etc then it might be worth a trip.
They have Graco Extend R129 for £75 at the moment, which is a good seat and belt fitted so light to swap. It can sometimes be outgrown earlier than the 105cm height limit, so would caution this may only last taller children until about age 3.
They also have their own brand Halfords Impala which is the cheapest RF seat you can get full stop - it's £60 (sometimes less on offer) and allows RF up to 105cm/18kg. Meets the R129 safety standard. I don't know anything else about this seat, not sure how big the shell is compared with the Graco or how solid it feels in person - might be worth checking out.
They also have Graco Turn2Me i-size for £129 which is a really good price for a spin, isofix seat RF up to 105cm (and decent space) from a decent brand, good safety result. Obviously still quite expensive x4, but might be more reasonable than some others. Because it's isofix, it is heavy to move between cars but it is simple to fit (tip: If the isofix won't release, push the whole seat IN to the isofix bars and then try to release again.) If you have the cars parked side by side with enough space to open both doors, it's easy to move it.
Graco, Nuna and Joie are the same parent company so there are similarities between their products although Nuna is the "luxury" side and Graco the "budget" side. Joie is somewhere in the middle.