You leave the 25kg seats in the car, you don't reinstall them every time you get the child in and out. Just the harness (which you'd have to do on every kind of seat!) If you do go for this kind of seat I'd look for one where you don't need to uninstall it to change the harness height as your child grows. That rules out Axkid Move, Britax Max Way (the Plus is OK), Britax Two Way Elite, Klippan Century.
It might be tricky to sort the installation if you have spatial issues and can't buy from a specialist, because I think there could potentially be a chance of misuse if for example you struggle with diagrams and might put something over when it needs to go under or vice versa. They aren't hugely complicated, so if you think you could team up with someone who is good at this kind of thing it might be fine. You'd never really have to do it again once it's done, unless you get a new car.
If you are really worried about it, you might want to look at the Joie 360 Spin. It's not i-size, so it's not limited to 105cm height, and it has a nice tall shell so it will last about 6 months longer than most seats of that size, but it's still isofix fitted so nice and easy to do. I'd get the German version if you can find it which is Joie 360 spin GT. It has a bit of extra side impact protection.
It might still mean that you end up with a high backed booster at a fairly young age but you might be OK with that? It seems to be personal. I personally wouldn't want to use one any younger than 3, but I know some people consider the minimum age should be higher than this.
Or, you might have a younger child once your older one is ready to move out of the spin seat in which case you could get something different for the older child such as a seat which is forward facing and harnesses up to 25kg (or even just 18kg if he's tall and slim), then turns into a booster seat later. Those tend to be fitted with isofix and top tether, sometimes seatbelt as well but with an easy belt path.