For me it's between the Aton M and Pixel from your list :)
All of the seats you mentioned go on the same adapters, so that's fine.
I don't like the Gemm, it seems to really scrunch babies up, and none of the newer versions have really addressed this.
The i-Level can only be used with the base, not seatbelt fitted.
Pebble 360 is really pointless to buy without the base (IMO). It's very pricey, you're paying for the rotation that doesn't work without the base, and you won't be using the Pearl 360 if you have a Minikid to move into.
Cybex Aton M was the longest shelled baby carrier when I measured them for someone who came into the shop I worked at. My only gripe with Cybex seats is they are heavy and they have a weird strap mechanism that's hard to tighten with the baby in. I just leave mine at the right length (get DH to do it) and slowly loosen as baby grows. It is a little bit of a pain to fit with seatbelt because there's a clip in the back slot which keeps the belt stuck in the seat. But you probably get used to this and overall it's a really solid, nice quality seat. I found it hard to find the right headrest position but easier with the second baby I used it for (the trick is to go lower than you would think).
Avionaut Pixel is also great. Lightweight, great position for babies, love the inserts. Only downside is that the underneath part is polystyrene, not plastic - I worry that it could get bashed around and damaged easily. Also, the sides seem shallow, although it gets a good side impact rating from ADAC. The seatbelt guides look really flimsy on it, too. Again, no problem in the crash test, but may be fiddly if you plan to belt fit often.
So between those two - if you want something sturdy and solid, then go for the Cybex. If you want something lightweight, and/or the newborn neck position is especially important to you, go for the Avionaut.
Don't choose a car seat based on being able to sell it on, the second hand market for car seats is very poor anyway, since advice is to avoid buying second hand. People looking for a second hand seat also may not actually know much about brands so I don't think the brand makes a huge amount of difference, unless you have something that is a household name e.g. Maxi Cosi Cabriofix.
Couple of other options to look at:
Cybex Cloud Z - nice and roomy, and has a recline option which is useful even if you don't get the base for the rotate option. Goes on your pram as well.
Britax Babysafe or Babysafe i-size are also worth a look, simply because they are roomy and they are a dream to fit with belt, so doesn't matter if you get the base or not. However only the Babysafe 3 / Babysafe i-sense work with Maxi Cosi pram adapters. The older ones (Babysafe, Babysafe 2) use Britax specific adapters which they (probably?) don't have for the Oyster.