@MyNameHasBeenTaken
I think part of the confusion here is the expectation of the isofix.
The stage 1 seat, with the 5 point harness is what the isofix is for.
Stage 2 (high backed booster) and stage 3 (booster) use the standard adult belt
So the seat is isofix compatible. It has the 2 metal clips on the rear of the base.
And it is a stage 1 2 3.
Stage 2 & 3 are generally not isofix .
I am not sure if there is a weight limit for isofix?
Yes, that's right. There's a 33kg weight limit for isofix harnessed seats, that has to cover both child and seat.
Booster seats the isofix is always optional, because the belt goes around both child and seat, so the isofix doesn't play a crucial anchoring role any more. It just makes the seat more stable, and means if you have a crash when the seat is empty it won't fly around and injure the driver/passengers. (In a perfect world, you should belt an empty, non isofixed booster in - in reality, hardly anyone does.) Officially, isofix on booster seats is called "isofit" but isofix is often used for clarity.
In the old(er) Groups system like Group 1/2/3, the 33kg weight limit effectively means you can only have isofix up to Group 1. There are harnessed seats up to Group 2 (25kg) under R44/Group system, but they won't be purely isofix, as the isofix system isn't rated for it - you won't get a seat weighing less than 8kg, that's also strong enough to hold a 25kg child in a crash. A lot of them are isofit - like booster seats - meaning they need to be belt secured as well.
With the newer i-size system car seats don't have weight bands any more, so for isofix they take 33kg, subtract the weight of the seat and the remainder is the child's weight limit. So for example you have Britax Advansafix i-size where the harness can be used up to 21kg, or Axkid One which can be used up to 23kg. These in-between weights weren't allowed under the previous Groups system, as the seat has to be Group 1 and/or Group 2, so up to 18kg OR 25kg, not in between.
OP that sounds like a great choice and I'm glad you've found a seat you're happy with :)