It sounds like he's bigger on the growth centiles. The Pearl 360 and most of the 123 type (1-12y) seats all have a similar issue in that the harnessed mode maxes out at 105cm or 18kg. If he's on the bigger side, that's no use to you as you'll need to buy another seat in a couple of years and you won't want to move him to the booster mode in a combination seat because he'll only be little still. Looking at the chart, if he'll be 75cm at 9m that's 75th centile which is borderline for the 105cm/18kg seats. It might be that he will slow down (DS2 did, just turned 4 and has plenty of space in his 105cm seat) but it can put you close to the line and then you're in an annoying position of just wishing they were 6 months older and not really wanting to buy a new seat just for 6 months' harness use.
Another issue with a lot of the 123 seats is that they are only forward facing. Since you've said you want to rear face for the next couple of years, it sounds like you're already aware of the benefits. Although there are some "all-stage" or 0123 seats which can rear face and then turn into a booster, I wouldn't bother with them. They make too many compromises, the rear facing mode is usually only allowed to be used in recline mode, they are absolutely enormous and impractical, and they get poor safety test scores. Get a seat for each stage, or that does at the max two things. Trying to fulfil 4 different roles (newborn seat, rear facing, forward facing, booster) is too much.
I wouldn't personally bother with the Pearl 360 either. £300 is a lot of money to take your base out of use. There are spinning seats available in the region of £200-280 which come with the base attached, allowing you to keep your base free in the event of needing it for another baby. It is a bit of a con this idea that you need the base for the next seat. Most of them don't need a separate base. At least compare it with other options.
So you need something to stay rear facing, at least immediately, and ideally to last longer than the standard 18kg/105cm. The next step up from that is 25kg/125cm. I'd look into the following options:
- Swedish extended rear facing seats, like Britax Max Way, Axkid Move, Avionaut Sky, Besafe Stretch, etc. These allow rear facing all the way up to 125cm (varying weight limits, but all more than adequate) - that might seem a bit unusual, but many children get on very well with this and the seats are comfortable and well-made, with good leg room etc and rear facing does continue to be of benefit right up to 4 years old and beyond. Only caveat is, as you have a 75cm limit seat and your baby will reach this before a year, it is worth seeking out a real life specialist retailer who stocks them to try before you buy, because unfortunately some of the cheaper models have a higher starting weight/height than the newer, more expensive ones. Look on Axkid's website for UK stockists to find one local to you (Axkid stockists often have the other brands too).
- Maxi Cosi Beryl. This is an unusual seat that is rear facing up to 13kg and then forward facing up to 25kg. Out of all the forward facing harness seats, it gets the best score from ADAC/Which, except that they downgraded it for side impact protection once the child's head is out of the shell, and the harness is notably short, so it likely won't get you all the way up to 25kg, but for your situation where your child is likely to be borderline 18kg at 3.5 - 4 years, this is really not an issue since it's not like you will need the whole 25kg capacity, just probably 20kg or so. You'll be able to use it as long as you want and then when he gets too tall, change to a booster. The cons are the low rear facing limit (comparative to 18kg/25kg seats) and it doesn't spin.
- BabyAuto Dupla - I know I said don't bother with the 0123 seats, but this is worth a look if you think you'd do a cheaper 0-4y seat for rear facing and then buy a 25kg harness FF seat later. It does both of those things (RF to 18kg, FF harness to 25kg) in the same seat so can be worth looking at. Cons are that it's a budget make, doesn't spin again, needs top tether and the safety performance is unknown compared with other seats, although rear facing is safe regardless of the seat and once you're in 25kg FF harness, there aren't many options.
Or you could gamble with the longer-lasting requirement, perhaps if you think it's likely you'll have another child to hand this seat down to later, it won't matter. Look at spin seats which rear face up to 105cm/18kg, and then when your next baby needs to move up from the baby seat, you can decide whether to move your elder child into a Swedish rear facing seat, or a 123 seat. This is a better time to look at 123 seats, IME, they do not fit 9 month olds well, but they do fit 2.5+ year olds well, and you'll know by that time whether you want to continue RF or change to FF, and if you want to FF whether your child is likely to have enough time in a 105cm limit harness (these seats tend to be better rated for safety) or needs the 25kg harness.
The benefits of moving to a spin seat now are obviously the spin function, the recline tends to be handy for a young baby who naps a lot, it gives you flexibility to forward face if you want to (or stay rear facing as long as you want to) and the seat will definitely fit from whenever you want to move into it, rather than having to wait for a minimum size.
Sorry that was a bit of an info dump. Hopefully it was helpful, feel free to ask any further questions!