Belted seat with no rear tether?

[FleurWeasley]

My son is approaching the weight limit for his 2way pearl. He's 3 years 3 months old. We would like to buy a car seat that offers rear facing for a bit longer and then be able to switch him to forward facing. The thing I am struggling with is a car seat that doesn't need the rear tether in forward facing mode - he has a diono radian 5 in our other car, but won't be able to forward face in that car as there's no tether point, so ideally we will switch that one to the other car (currently has a 2way pearl in) and buy a new one for the non-isofix non-tether point car.

The britax 2way elite would have been ideal but the forward facing harness now doesn't go past 18kg!

I would go with the tactic of finding which seats harness past 18kg (as I don't think there are many) and then check if they rear tether.
I was going to suggest the Joie stages but the harness only goes up to 18kg.
I'm not sure you will actually find what you want, I think it may be the heavier weight limit for the harness added to the heavier weight of a multi group seat which necessitates using a rear tether, if you see what I mean? Though i know the britax didn't (but now has the different weight limit).
If you keep him rear facing you'll have no problems keeping him harnessed til 25kg - there are lots that do that as you probably already know.

sorry my first sentence should read "which seats harness forward facing past 18kg..."

Thanks moggle, that's the conclusion I've come to! Will look into getting a tether point in the car.