For the next stage up, you don't usually get separate bases, the seats either have isofix built in or they don't and are seatbelt fit only. There are a few exceptions, and if you already have a Maxi Cosi FamilyFix base, then there will be a next stage seat you can buy to go into it (check compatability here: https://www.maxi-cosi.com/c/international/base-compatibility ) however IME these are usually just as expensive or even more so than the isofix seats without a base, or the 125cm limit ERF type seats.
Because the seats for the next stage are much heavier and they generally have some kind of anti-rebound bar, they won't move up and down like a baby seat base can do. But as your retailer said, this is totally normal and not a concern anyway. In an accident, the force would be making the seat/base rotate forwards and down, so the leg acts as a brace and prevents too much movement in this direction as seen in this video. Rest assured BTW, this is an American seat and they have a different belt routing. UK seats don't move this much because the seatbelt goes around the back, but it illustrates the job of the leg very well.
To be honest isofix seats usually take up more space than tethered + seatbelt fit ones. So if space is a concern, then the longer-lasting seats are actually great for that aspect too.
The kinds of seats people usually go for for the next stage if they want isofix for a mid range budget would be things like Britax Dualfix M Plus, Maxi Cosi Mica Pro, or Cybex Sirona Gi i-Size. Joie 360 Spin is also popular for a more budget friendly option.
Bear in mind that these don't last as long - usually up to 18-20kg and 105cm. Whereas the 125cm limit seats have much higher weight limits of 23-36kg. You can get seats which promise to be usable up to around 10-12 years, but these incorporate a booster seat mode and honestly, this just makes them more bulky and you get compromises in performance in each mode, so it's better usually to get a seat just for this next stage (rear facing, some also include forward facing) and get a high backed booster when this seat is outgrown.
How big is your LO now and what age are they (in months)? It can be useful to make a rough estimate for when they might outgrow a 105cm limit seat vs a 125cm limit seat. For example if they are very tall then it can be worth the investment for a 125cm one so that you don't end up needing to buy two.