The 125cm limit seats need tether points attaching to the rails of the front seat. That is a pain to do every time, because it means grubbing down on the floor under the seat, so if you have a second car that you will use the seat in, what people usually do is buy a second set of the tether points (you get one set with the seat but they are about £20 separately) and install those in the second car. They are just straps so tuck away under the front seat when not in use.
In general though, the seats for the next stage, whether you get ERF tethered ones or isofix spinning type ones are a pain to move around because they are so much bigger and bulkier than the carry type seats that younger babies have. So if you're likely to need to move it with any regularity, most people do find they prefer to have one seat in each car. Of course it is possible to move seats in and out, and they can often be fitted within about 2-3 minutes once you get the hang of it, but the reality of it day to day in all weathers, with a toddler in tow who can run around or might be tired and throwing a tantrum, is just not much fun - so think about when and how often and why you'd do this and what the scenario would be. IME, seats for the next stage are so heavy and large that you can't easily hold them in one hand (apart from the cheap, belt fitted ones).
How old is LO and how big is your infant carrier? ie, does it go up to 75cm or something bigger? I would not overly worry about the seat being suitable from birth since you should be able to reuse your infant carrier for DC2. Seats like Movekid which are suitable from ~6m are fine for this. You may even be OK with the older Minikid (version 2) or Move by Axkid, which are suitable from approx 12m+ depending on the size of your infant carrier and your LO's current size.
Equally though, if you plan to have a second child then the need for a 125cm limit seat immediately is not that great because if she does end up outgrowing a 105cm limit seat (around 4.5 is average for 50th centile) and you're not yet happy to go to forward facing HBB, you could then get an ERF seat for her and the younger child can inherit the 105cm limit seat (if they haven't already). Bear in mind that Avionaut Sky is around equivalent to a bigger 105cm limit seat in size, usually accommodating up to about 110-115cm in height.
If you're happy to spend up to £339 for the Avionaut Sky, then it is worth looking at 2x Britax Safe Way M which works out at about the same price, if you can still find it on discount. (There are other seats around the £200 mark, so you can get 2x ERF seats for about £400 in total).
Or if you had around a £250 budget for the more-often used car, then you could get a seat like Axkid Move, Axkid Minikid 2, or Britax Swingfix Plus, and then for the less-used car, a cheaper belt fitted 105cm seat with RF option around £100, like Graco Extend R129 or Joie Stages, or Graco Slimfit R129 (if you are trying to avoid any FF mode). These are also a bit more lightweight so can be useful as travel seats, seats to move around for grandparents' cars etc.
It is worth comparing Axkid Spinkid to Besafe izi Twist. That is another plus tested spin seat. It's heavy so not that easy to move, but it does have a brilliant levelling feature which allows the seat to be angled independently of your car back seat angle. So that can be excellent if you find your back seats tend to tilt isofix car seats backwards. Currently on offer for £309, and you may even find it in stock near you depending on what stores you have nearby. It feels unbelievably sturdy.
If budget is not fixed, you might also want to look at Axkid One - the older version One2 is available at some places for a discount price of £435 - which I appreciate is higher than the seats you're currently looking at, but this one is apparently quicker and easier to move between cars because it is isofix (so no tethers) and lighter than the spin seats. It also has some other nice features like sliding rails so you can easily adjust the leg room for your child vs creating more space for the front seat. Really nice seat, admittedly pricey. I believe John Lewis sell this model as well, so again you might be able to find it in stock somewhere to look at.