Is your Cybex seat one which goes on a base? If you haven't washed the base as well then it might make sense just to look for a new matching seat for the base - just double check compatability. There is a Cybex Sirona G seat on offer at John Lewis at the moment which goes onto a base, though it might not be the same base that you have.
If you want to look at new seats anyway:
The only rotating one which would really make sense is the Besafe Beyond, because it goes up to 125cm rather than the 105cm that most rotating car seats go up to. But although you said you're not bothered on price, it is a very expensive one - it needs a separate base so the total cost is about £620. That seat would last until he is about 6.5 if he stays on the same centile line. It goes rear facing and forward facing. Excellent ADAC score recently - "Very Good" for safety, which is not an easy score to get at all particularly for a seat that accommodates up to age 7 (ish) although bear in mind the ADAC test was on the rear facing only version. If you got the 360 version which also allows forward facing, the safety might not be as good for the forward facing mode. After this seat you'd need to purchase a high back booster (HBB) which is the last seat type you need that takes him up to about 10-12 years when he can just use a normal seatbelt. These are much cheaper, about £100-150.
Or you can get a purely rear facing seat up to 125cm and either 25kg or 36kg for about £200, these seats would last him to about age 7.5. Look at Axkid Move, Axkid Minikid or Britax Safe-Way M. They don't rotate, but they are very comfortable for older children rear facing - much more leg room than Cybex, and are basically the safest seats on the market as they have all passed the Swedish Plus test (which is even better than a Very Good ADAC score). Again you'll need a HBB after this.
If you want something which will convert into high back booster, you could look at rotating all-stages seats - the best performing one of these on the market is Silver Cross Motion All Size. It's similar to your Cybex seat up to 105cm/18kg (around 4.5 years for your LO), then it converts to a high back booster. Cost is about £300-350 depending on sales. You'd retain the ability to rear face up to that age, the rotation, and you'll be able to keep it to convert to HBB for later so you won't need to buy another seat. The only downside of this one is that the safety rating is less good than having a seat exactly like your Cybex rotating seat (which in turn is less good than the rear-facing-only seats mentioned above) and less good than a typical high back booster from a good brand. And the safety rating of all-stages rotating seats from other brands are even lower. You'd also be stuck with a heavy, bulky seat for the majority of the time you'll use it as you'll only use it as a rotating seat for about a year, then as a high back booster for potentially another 6 years or more - if you got another rotating seat now, perhaps a cheap one like Graco Turn2Me, and then changed to a separate high back booster after this, then you'll get a much more lightweight seat which will possibly also fit better and be more comfortable as he grows.
One last option to consider is a forward facing seat which converts to high back booster. Personally, I'm a fan of the Britax Advansafix Pro, but Silver Cross Balance i-size or Maxi Cosi Titan (any of the i-size variants) are also good options. These all harness up to 105cm and then either 18kg for the Maxi Cosi, or 21kg for the other two - again around 4.5 years, then you tuck away the harness and convert to high back booster. Not as bulky as the Silver Cross Motion All Size, cheaper, and hard to judge exactly, but it's likely that these seats outperform the forward facing and HBB modes of the Motion All Size. Of course, the downside is that you lose the rear facing option, and you're right that rear facing is still safer and still beneficial, though at 3.5 years it's also a totally reasonable option to forward face.
Personally in your case I wouldn't opt for a 25kg limit forward facing seat like Joie Bold; you don't really need that extra harness capacity if he's forward facing anyway, and the ones with 105cm limit get better safety ratings overall.
One last note, although LO is just at the minimum stats to use a forward facing high back booster type seat (100cm/15kg) and some are marketed as being from age 3/3.5, these only use the adult seatbelt to restrain the child, so it really is better to keep him in a 5 point harness and/or rear facing seat for now until he's closer to 105cm/18kg - this is probably about a year's time. If you did opt for one of the 125cm limit options, then you get the benefit of rear facing/5 point harness for even longer.