I don't actually think the "too many categories" thing applies to 0-18kg ERF seats (happy to elaborate on this). Some of the original spin seats to come to market were quite short in the shell, but newer ones are taller. Some can sometimes have a poor fit for tiny babies, but actually, so can a lot of infant carriers - and I've seen some newborns fit much better in larger, spin type seats with inserts than many do in a lot of infant carriers, which tend to be quite curved. Babies are just so tiny when they are newborn and grow so fast that it is really difficult to get a very good fit for a newborn in a car seat in general. (The best fit I've ever seen for a newborn in any seat ever is the Avionaut Pixel, which is Venicci Ultralite for the UK market, if you can't get the Pixel where you are).
When choosing between different models of spin seats I'd look at:
- Max headrest extension/highest harness slot (longevity of seat)
- Fit and stability in your car - check in all modes so reclined, headrest extended, newborn inserts in/out etc
- Recline angles (suitable for non-sitting baby/curious toddler?)
- Leg room for older child
- ADAC testing for side impacts
- If seat has possibility to RF and FF and there is a RF only model, ADAC/Plus testing for that for frontal impact (will tell you relative safety of seat when in RF mode)
- Ease of adjusting harness
Since you'll use this seat for a long time it's well worth visiting a car seat specialist if you can possibly find one (I can recommend a couple of chains in Germany?) - if not, your second best bet is a nursery specialist store, preferably one which advertises that they offer car seat advice (usually their staff will be trained by car seat company reps) and are happy to let you try several models in your car to compare ease of use and fit etc. Online might be cheaper, but it's worth spending a little bit more to get this expertise. Since you don't want an isofix base for the infant carrier, it also frees up a bit of money to put into your decision for the right "main" seat.
The only other possibility I would look into before looking into the different spin models is whether you think you might one day be interested in a 25kg limit ERF seat. After your baby grows out of the spin seat, you will then usually be looking for a high backed booster seat. If they are four or five at this point, then this might be something you're absolutely comfortable and happy with. But if they are two or three at this point, you'll probably not be happy with it at all, in which case, you'll then be looking at larger rear facing seats, and the next bracket (which is the largest available) go up to 25kg. Sometimes people also choose a 25kg limit seat even if they have smallish children, simply because the difference between rear facing and forward facing is the biggest change in safety in the car and they just want to delay that change for as long as possible, which is also reasonable. The reason I say this is that 25kg ERF seats tend to cost about as much again as the spin seats themselves, and if this is something you're likely to want to go for in the future, you would be best off with a long lasting infant seat (with base if you want, but you don't need one) to use both at home and for travel, and then change to the good quality 25kg ERF for at home.
You could also just buy an infant carrier for now (and use for travel later) but narrow down both a spin seat and a 25kg ERF seat you like, and see how big/small/average your baby is by about 12 weeks, and make the decision based on that. Or if you are very likely to have a second child, it's a fairly safe bet to go for the spin seat and delay the decision about spin/25kg until you need a second seat covering Group 1 anyway.
As far as travel seats go, I'd get the longest lasting, most lightweight infant carrier you can find in budget, bonus points if it fits onto a pushchair and/or is flight approved and use it until it's completely outgrown, which could be a good 18 months (my 75th centile first child lasted 18 months in his) or even 2+ years if you have a smaller than average child. Then you would have a few options to follow on from this:
- Britax Eclipse. Forward facing only, so not great for safety, but can be fitted with a lap belt or normal seatbelt, is flight approved, is lightweight as Group 1 seats go. Switch to Trunki boostapak as soon as you feel this is acceptable for child.
- Britax Two Way Elite. Rear facing up to 25kg, so max safety, but larger and more bulky to carry around, although the lightest and most universal of all the 25kg ERF seats. Flight approved. May be pointless if you plan to swap to a booster ASAP.
- Any lightweight 123 seat - forward facing, but most lightweight harnessed option, and will last until booster stage. Better than nothing, but the very light 123 seats tend to be extremely light on safety.
I wouldn't bother with the Doona personally - it just seems silly money for an infant seat, most of which already have the possibility to go onto wheels as a pushchair, although you can find people who absolutely adore this seat.
However I will slip in one last wildcard - have you seen the Nachfolger inflatable car seat? That is a 0-18kg rear facing one. It is surprisingly solid (I have seen/handled one IRL), supposedly suitable from birth with an insert (probably not for tiny newborns, but assuming you wouldn't be travelling for 2-3 months after birth) and supposedly solves your exact problem. It is about €300, but at least where I am, it's cheaper than the Doona, and it would mean you wouldn't need an infant carrier at all. Plus, no worries about the seat sustaining damage in a plane hold, since it would be just a deflated shell, and the crash test results are actually alright - not the best seat in the world, but actually much better than most of your other realistic travel options. If you're going to travel a lot it quite possibly would be worth the investment, or at least looking into it.