So am I right in understanding that when he gets to 75cm, or 13kg we should move to the next seat, even if say he is 75cm but weighs 11kg?
Yes, that's right.
Maxi Cosi don't usually get into the lists of Axkid, Britax, Besafe because they only do seats up to 105cm (approx age 4) and as you know the ERF pages prefer to recommend RF as long as possible, so they just go straight to the longer lasting seats. It's not that Maxi Cosi are bad, it's just that if you want to RF as long as possible they don't do seats that do that, and if you need a cheaper seat and don't mind about it not lasting so long, Joie do that better.
Besafe Stretch would be fine for using any time - you could even use it now if you wanted to. It's a good seat, one of the few of the up to 6 years that you can recline while it's installed. The others, you set the recline according to the age of the child and leave it.
Minikid 2 is the older regulation which is why it's more for use from 80cm (ish) - these seats tended to be designed to be either from birth or from 9 months, and Minikid was a from-9-monther, even though it's approved from 0kg - it's a long story.
With the newer regulation that came out in 2018 or so, you can have seats that are approved from birth or from 61cm (approx 6 months) and so the Minikid 4 is the newer version of Minikid 2, but it's quite a significant redesign being suitable from 6 months right up to 125cm (which is around 6/7 years) and 36kg, instead of the 25kg limit of the Minikid 2.
Minikid 4 also has easier installation.
You might see generally stated that the older regulation R44 is "less safe" than the newer regulation R129. This is sort of true, but not really applicable to the Minikid 2 which was already one of the safest seats you could buy. So the main benefit of the newer regulation for Minikid 4 is the fact that the weight and age span is different, which wasn't possible under the older regulation.
Minikid 4, Britax Max Space, Britax Safe Way and Besafe Stretch are all what I'd think of as the "newest generation" of ERF car seats with the Minikid 2 being from the previous generation. All of this newer generation have clearer/easier fitting, more automation in the tethers, fitting from approx 6 months and use up to 125cm/36kg.
The ASIP and SICT are just different trademarks for the different brands' side impact protection systems. Side impact protection is important in car seats as if you were hit from the side (T-bone) you want the child to be well protected. Many older car seats did not have good protection in this area, as they had focused mainly on frontal impact, and it has been a focus in the car seat industry, especially among safety focused brands, for probably the last 20 years or so. So among a lot of the top brands (Britax, Maxi Cosi, Axkid, Besafe, Cybex etc etc) they have each developed their own terminology and trademarked terms for the different systems that they have. All the seats you're looking at have these features, except I think the Britax Safe Way M is a more stripped down version and doesn't have the extra block on the side - it's not essential, but it is nice to have.