I was going to suggest the Cozy n Safe Hudson as a more budget friendly model, so glad someone else has suggested it. Boots have it too and they have some kind of discount on top. Again it needs to be the old version not the i-size.
If you want truly escape proof there is Cybex Anoris, which uses impact shield/airbag rather than a harness, and is suitable up to 115cm and 21kg, but I don't think it's a great solution given that it's about 4x the cost of the Joie seat.
Some people have pointed out that there are loads of seats which go up to age 12 - the majority are no use unfortunately, unless they have a 25kg harness because the harness mode on most of the "up to 12" seats has a max of 105cm/18kg (ish) - this is the size of an average 4.5 year old, and the same size as the OP's DS's previous seat which he has nearly outgrown.
A high back booster seat is a perfectly acceptable, reasonable seat for most 4yos. Some will need the harness a bit longer, which is fine. And yes you can RF longer, but it is an expensive outlay if it doesn't fix the issue, and it seems unlikely that it will. Unless you can catch them at a rare moment of zen and manage to convince them it's really exciting to go backwards.
OP I am loathe to offer parenting advice since you've received so much unsolicited but you might like When The Naughty Step Makes Things Worse, it's only recently come out so you might not have seen it before. Seems useful for DC who are truly resistant to everything.