I think I saw this thread at the time and sent a thanks to FamilyAreEverything as she said everything I would have said! :)
Tethers don't need to be fitted every time. You install them usually around the front seat rails, then they stay there and they can stay fitted. Even if you take the seat in and out of the car, the tethers can stay in the car, they just tuck under the front seat so aren't in the way, then they click in similar to a seatbelt buckle. If you don't take the seat out of the car, you don't need to touch the tethers at all.
I understand that they are relatively simple to fit in most cars, but if you have trouble or don't want to do it yourself, if you can find a local ERF retailer, most of them will fit the tethers for you as part of the service.
If you look on the Axkid website, they have a retailer search, so you can see if anyone is near you. Most Axkid retailers also sell other brands, so they can show you all the options at all the different price points. Because these seats have a higher than 18kg weight limit, you'll be able to use them much longer than a standard seat - up to 5 years even for the very high centile children, up to about 6/7 years for most children, some even longer.
If your LO is on higher centiles, then I would probably not use the Stages for him forward facing. After 18kg, it is only allowed to be used as a high back booster, and honestly it's not a great high back booster, it's OK, but the fit isn't as good as some others, and it will be outgrown at about 125cm at which point you'll still have 10cm left before he can legally go without a car seat (or 25cm until it's recommended) so you will need to buy a full-sized high back booster for him long term anyway. IMO you might as well bite the bullet and buy it when he first goes into a booster, rather than using the stages for a couple of years and then upgrading.
One issue with higher centile children is that for all the belt fitted seats, the forward facing seats max out at 18kg, although I think there are a couple with 105cm/21kg combination on the harness. So if you want to keep him in a harness past 18kg (which would be recommended if he is still under 4) then the only option is normally rear facing.