The thing is you can't really say "It was fine..." unless you were actually in an accident then you don't really know whether it would have been fine or not. I guess you could say look, he was fine in his RF seat in the accident that we recently had, so I want to stick with that. It's always percentages anyway, not a guarantee. But certainly for a child under 1 I would want them RF. At least until 15/18 months, preferably longer. But that period around 1 year they are especially vulnerable, even in lower speed crashes. It used to be when EU seats were all 9kg to move up, when you looked at pan-EU traffic accident stats, you would see a clear spike in the fatality numbers of children around 12mo because they were being moved to FF and not surviving in accidents where they would have survived RF. Younger children were mostly RF so were surviving. Older children it's more the fringe cases where it makes a difference. This is a large part of why the newer guidance was changed to minimum 15 months.
The smallest rear facing seats are the 25kg ones or possibly the Joie Tilt I think takes up less space than the Joie spin seats. The multi stage ones like Stages / Every Stage take up a lot more space than other RF seats because they need to be so reclined, plus a bulky base, plus the height needed to expand for older children. Britax Max Way is the cheapest RF 25kg seat that is relatively compact. (Though it is about £200, so I appreciate not cheap).
Another couple of tips for fitting a RF seat are to look at the recline of the front seat, could you comfortably make this more upright? That can often get you a few more cm. Are you needing the front seat to be further back to accommodate a tall driver/passenger? Could you fit a belt fitted seat in the middle? Or in the front with airbag off? (Even temporarily until you feel DS is old enough to FF safely?) This is an article with album link at the end showing ERF in many small cars. The tone is a bit snarky but if you can ignore that and look at the actual car seats it is helpful. You will notice very few isofix spin seats because they tend to be bulkier, only one multi-stage seat and not many Joie seats except Tilt. Some of the seats here are older models/unavailable now but there are updated versions of most of them.
erfmission.com/you-cant-rear-face-in-small-cars-debunking-myths-4/
Which Joie seats do you have now which fit FF but not RF? TBH, I would be reluctant to use a Joie seat FF for a child that is younger, because you must take all the inserts out to use them FF, and it tends to mean they don't fit that well. And the belt fitted ones have very minimal safety features FF. They obviously pass the legal testing, but I'd be happier with something like Maxi Cosi Tobi that has belt tensioners for example, or something like Britax Trifix or Maxi Cosi Titan with top tether.
As for the impact shield seats, it's not really true that no car seat expert endorses them, certainly people who are very pro ERF do not like them, and they don't really compare to RF, but if you're not comparing against Plus tested ERF seats but a standard FF seat, they can actually be a reasonable option for FF.
The problem with the ADAC/Which ratings on the impact shield seats (airbags or not) is that you're really only comparing the neck forces. It's true that they do better than standard FF seats for neck forces. (Though not as good as RF). The airbag one does especially well with neck forces, basically on a par with RF, which is encouraging. The piece of information which is missing from these test results is the ejection chance on rollover or offset crash (where you crash head on, but don't hit each other straight, e.g. when one car is only slightly over the centre line) and the potential for abdominal injuries. These risks are not tested for in those crash tests. Standard impact shield seats in the same style as Cybex Pallas, Joie Trillo Shield, the Kiddy seats (now defunct) etc, have been around long enough to have real world numbers on their use and when everything is averaged out they have about the same average protection against serious injury/death as forward facing 5 point harness seats. Fewer neck/spine injuries, higher ejection and abdominal injuries. The Cybex airbag seat is NOT included in this comparison because it has not been in existence long enough so we don't know if the airbag alleviates any of the issues. There is a slight advantage of shield seats for younger children (around 1yo) but the advantage tips to 5 point harness by the time children are around 3yo. Whether that's to do with them being less vulnerable to neck injury by that age or whether it is to do with them being that much larger with less of their body behind the shield, I don't know. Nobody has studied that.
To be perfectly honest if you have the £700 (!) to spend on the Cybex Anoris I would look at spending it on the Axkid One2 or changing the car.
To summarise:
- Try a 25kg seat such as Britax Max Way or Axkid Minikid if you can find a local stockist
- Try Joie Tilt (though it will probably only fit until ~2.5, at which point you could switch to your FF seat)
- Try the RF seat you currently have (if belt fitted) in the front or middle seat (airbags off in front).
- Swap cars with DH until you can change your car
- If going for FF, go for a highly rated one with excellent tight fit for LO and good safety features
- Consider impact shield seats as well
Lastly I've just gone back to OP - outgrowing at 70cm, are you sure of that height/that he's outgrowing it? That would be a very unusual height to outgrow a silver cross seat as they are fairly roomy. Have you removed all inserts, and do you know about the compartment on the back to raise the straps (if it's that SC seat that has it)? How are you classing outgrowing - head or legs? It's OK for a baby's legs to come out of the end but their head must be contained inside the shell. Sometimes people worry about it when there is about 2-3cm of growth left, but babies do stop growing quite so rapidly, that's why larger clothing sizes go up every 6 months rather than every 3. You might feasibly be able to wait it out. As others have said, the car seat should be discarded after a crash, but you might be able to get something like Joie Juva for instance, which is a cheap but roomy infant seat, fits with seatbelt or optional base for about £60, and keep him in that for a good 6 more months to tide you over.