What do you feel confused about? Overall, those three are all good seats and any of them would be a good choice.
To be honest, I'm a bit out of date on the exact differences between the newer Cybex models, and I haven't seen the new Rodifix Pro in person. For Cybex - I know that the T is the super-duper best top of the line one that they do - it expands outwards as they grow (which can be good for boys especially if they are broad across the shoulders) but the G is also a really great seat and can be significantly cheaper. It doesn't seem to be at the moment, though. (For the T, the "Plus" version is literally just different fabric, more denim-y, maybe a bit less sweaty compared with the cheaper fabric).
You might be able to get the slightly older versions Solution Z i-fix (equivalent to the T) and Solution S2 i-fix (equivalent to G) a bit cheaper.
Did you get your DC to sit in the different seats? Which one did they find comfortable? Did you have a good sense of where the belt sat on them, particularly at the hips, and did you try any in your car? (Seatbelt/headrest position can be a helpful deciding factor too).
Maxi Cosi used to be shorter than Cybex if extended to full height - this is worth checking if you want to keep using the seat all the way through, OTOH if your car has sloping ceilings at the edges, you might not actually be able to extend any HBB all the way up anyway. I don't know if the newer Rodifix models have this issue (the Kore ones did).
The expansion of the Solution Z/T can cause an issue if you ever need to sit someone next to the car seat as it takes up more space. If you need slimline seats for the space, Maxi Cosi tend to be good for this.
The Discovery is a good seat, but it's not one I'd suggest for comfort specifically. We had the older version of this (well, the non-isofix version Adventure) and my older DS used to find it uncomfortable for longer journeys, but it was a great budget model for day to day. And yes he used to get pins and needles and we discovered a suitcase works well, but there are proper footrests you can buy.