There is one more forward facing seat with harness up to 25kg that doesn't use top tether, Maxi Cosi Beryl. But it's about £300! And not that tall in the harness apparently, so I'd want to see it "in the flesh" before you buy and check how much growing room you have. Top tether isn't a bar under the seat BTW, but if you don't have isofix it's very unlikely you'll have top tether anchors.
High backed boosters again the max height on them varies. At 4 this obviously won't be a pressing issue, but if you want to use it right up until he can use the adult seatbelt safely, then it might be something you want to bear in mind. Otherwise you're looking at changing to a backless booster once their shoulders get too big to fit under the headrest, or buying another HBB once you get to that point.
Diono is a great choice as their Monterey one gets great safety testing results (which means their other ones most likely do as well) and they tend to be very tall, some of the tallest on the market. There is a cheaper one called Cambria 2, if you want. I don't know what the difference is between them, you could try contacting them via email or social media.
Don't worry about isofix on high backed boosters, BTW. It is always just for stability and can be folded away.
One feature you might like to look into with high backed boosters is the ones with a hook or loop that goes over the lap belt to help keep it in the correct position and reduce the risk of the child submarining under the belt. I think there are 3 on the market at the moment - Besafe iZi Flex Fix, Britax Kidfix series, and Chicco Fold & Go isize.
The Britax Kidfix ideally should be checked in store because they cause issues with the seatbelt retracting in some cars which is a safety issue. However if they fit correctly they are some of the highest scoring seats for safety on the market at the moment.