You do need the base as well for the Besafe Beyond if that is something you haven't factored in.
ADAC release their test results every May and October, so if you can hold on a month or so, the Stardust might be included in the next round. I have no idea whether it is or not. And it will be difficult to compare the result directly to Besafe Beyond because it is not the same category - ADAC average out the results in every use mode, so a lower result (expected) for the high back booster mode or forward facing harness mode will lower the overall score, even if the score for rear facing mode is good.
Have you had the opportunity to try any of the fixed/non spin seats like Besafe Stretch or Axkid Minikid? That is worth doing, as the spin is nice to have, but many people find that with the rear facing seats they are low down and open and so they don't really miss it.
Another option would be to go for the Sirona for now, although this doesn't have the Plus test either, or another 105cm limit spin seat (some of which are plus tested) and wait and see what age your child is when they are close to outgrowing it. Children do not always stay on the same centile line. If they are too young when they reach the limit, you could then opt for a cheaper, fixed rear facing seat without spin.
In terms of different safety standards it all just gives you different information. In reality, any rear facing seat which has passed any regulatory standard is going to offer excellent crash safety protection. It could be the most basic R44 certified seatbelt fitted seat and this would still apply because it is basic physics. It does seem that having a support leg is especially protective compared with other methods of reducing rebound such as a seatbelt or top tether, especially with seats up to 125cm.
R44 vs R129 tells you that at the very least, an R129 seat has some kind of side impact protection and will be easier to use in terms of adjusting the 5-point harness, which is good because a wrongly adjusted harness can be dangerous. The rules for things like how much the dummy's head is allowed to move in the crash test are also slightly stricter, but this doesn't really matter for rear facing. It does matter for forward facing seats.
ADAC is generally useful in that you can clearly see seats which are purely rear facing perform much better than seats which are forward facing or used both ways. And because the crash test is performed at a higher speed, it is useful in that occasionally a seat will completely fail at this higher speed, which is concerning. These are usually cheaper/generic seats, but in any case if it happens the company often recall the seat. So to have a seat have a good ADAC rating gives you extra peace of mind, but IMO it isn't necessary - though I would certainly avoid seats with a bad ADAC safety rating, I would and have used seats which don't have an ADAC rating at all.
Plus testing is IMO largely superfluous. It is basically an instrument to show that rear facing is much safer than forward facing. Realistically there is no difference for example between the Britax Swingfix, which is plus tested, and the Britax Dualfix - they are both spin, isofix seats up to 105cm and the only difference is that the Dualfix can be used rear facing and forward facing with full 360 degree spin, whereas the Swingfix is locked to 180 degree, rear facing only. They are produced on the same factory line and the only difference is this lock on the Swingfix is permanent whereas in the Dualfix you can switch the lock on and off depending on the age of the child.
That is not the case for all seats - for example, Joie have the 360 Spin and i-Spin 360 which are both rear and forward facing spin seats, and in order to certify their Spin Safe and i-Spin Safe seats for the plus test, they had to modify the seats slightly, which suggests that if you use the Joie 360 Spin in purely rear facing mode, it will still offer good protection because it's a rear facing seat but the performance is not quite as good as a Plus tested rear facing seat.
I've just been to check, because I kept hearing that the Stardust does have a rear facing only version which is plus tested and it seems that is now official - the Stardust RWF+ model is plus tested, however, it is limited to 105cm, and does not convert to high back booster. Whether that means the 125cm limit version doesn't quite meet the plus test criteria, I am not sure. I know that seats which have a forward facing mode are not allowed to be tested.