The thing about the R129 standard being "best" is a little bit misunderstood.
Basically, all car seats, being an essential safety item must meet a minimum level of safety. You know like there are safety regulations for all kinds of items - riding helmets, climbing harnesses, mattresses, cots, CE mark on toys, kite mark on condoms, even components of cars themselves.
Children's car safety seats are the same, and we've recently (ish) had an update to the current regulation. It used to be R44.04 and now it's R129.
R129 is more up to date than R44, which means that it's a bit more stringent and a higher standard. If you have a choice between an R129 standard seat and an R44 standard seat, it might be that the R129 seat is better (though, honestly, not always). However these are just the minimum safety requirement. What they basically say is that any seat that doesn't meet these standards is so unsafe that it's illegal to use or sell it.
Because R44 is older, there are very few seats left on the market approved to that regulation. So saying "Has R129 - safest standard!" is a bit like marketing a car by saying "Contains seatbelts and airbags!" - YES - those are important safety features. But they are also very, very standard, and nothing to get excited about.
People are talking about ADAC because ADAC is the German version of AA (motoring club) - twice a year, they crash test a selection of children's car seats and sell the results all around Europe to various publications. In the UK, they are published by Which? - Which? add their own criteria and commentary on the results, usually for things like user-friendliness etc. So if you want to know how good a car seat is compared to other car seats, ADAC or Which? are good places to look for info. They have safety ratings "very good" "good" "satisfactory" (OK) "sufficient" (barely passes) "insufficient" (failed).
In addition to this, as you've found, there is a history in Scandanavia of rear facing car seats - just happened to be that way, most developed countries independently developed car safety seats for children in the 1970s, most of them forward facing just as the adults are in the car. Sweden by some chance the first designer was inspired by rear facing astronaut seats, and developed their rear facing child car seat in the 1960s - this stuck. In the mid 80s, child seat standards were developed in North America and Europe where child seats had to conform to that safety standard. Because Sweden was outside of the EU at this time, they kept their own independent safety rating. When they joined the EU in the 90s, they had to adopt the European safety standard, even though it was technically lower than their own standard - so they developed an optional, consumer test that companies can put their seats through to show that they meet the higher standards of the old, original Swedish standard. This has also been developed and refined over the years and is called the Swedish plus test. Plus test is considered the highest marker for safety, better than a "very good" ADAC rating. However, these seats are all rear facing, so no use to you if you need forward facing seats.
I would personally in your case look for brands/models which have performed well with ADAC (Britax, Maxi Cosi and Recaro are all good for forward facing) and avoid the brands which tend to do less well with ADAC (the generic brands, of which Cozy and Safe are one, tend not do do so well here).