You probably won't get long enough out of a swivel because of his weight centile - there is the Besafe Beyond, which goes up to 22kg, or Avionaut Stardust which is up to 21kg, but these might not even get you to age 3 (which is too young to switch to high back booster) unless he drops some centiles when he starts walking, and they are pricey seats to have to replace so quickly.
Since he needs to stay rear facing at his age, it would make a lot of sense to look at the rear facing seats with 36kg weight capacity, so Axkid Minikid 4 or Britax MaxSafe Pro or Britax Safeway M. As PP say, the sides on these seats are low down so you don't "miss" the rotation as such.
The older minikid with 25kg limit is cheaper, but at 7mo it's likely he will be too short to get a good fit in it - most children fit into it starting around 10-12m. If he's tall it might be worth a look, and any stockist of the Minikid 4 might have the older one too, so you could compare.
The other thing you could do is get a cheap-ish swivel seat (one or other of these is often about £200) with 20kg weight limit - either the Cybex Sirona Gi i-size, or Britax Dualfix M Plus. Knowing that they will probably only last a couple of years, but at least they are cheaper than the other options and you get the swivel function for those couple of years.
(I realise you already have the Cybex Base T, but the Sirona T is more expensive and has 18kg weight limit).
Then once your LO is close to 20kg, you could decide whether you want a cheaper 25kg/36kg ERF seat to keep them rear facing, or look at an extended harness forward facing seat, most of which also go up to 25kg.
Just bear in mind if you do end up going for a cheaper ERF seat later, one of these is the Britax one which would already be suitable, so it might make sense to just get now.