Joie Elevate R129 or Graco Eldura R129 is belt fitted and goes up to 21.5kg / 105cm on the harness, you can physically fit them into the harness longer, I am not sure if it's advisable to go over the height limit or not.
In fact, the Eldura manual and website explicitly says you can use it until the child's shoulders reach the highest position:
Website:
Your child can stay in harness booster mode as long as their shoulders comfortably fit when the harness is at its highest setting.
Manual:
DO not use the booster with the harness if the child’s shoulders are above the highest slots in the backrest or if the child’s weight exceeds 21.5kg.
The Joie manual says the same, but doesn't have the line on the website saying it's OK to use as long as shoulders fit.
If you use one of these seats, the belt path is fairly shallow, meaning there is a danger of what's called forward anchorage on the front seat in some cars, if you move the seat back too far. If the seatbelt anchorage points (where the two ends of the lap belt attach to the car) are physically further forward than the belt path in the car seat, the seat will be loose and it will be impossible to get it tight enough. If this happens, move the front seat forward just enough that the belt is anchoring the seat back and down firmly. You still want to have as much distance as possible from the dashboard. You can kneel into the seat to pull the belt tight, but you don't really need to because this seat has a belt lock off. So yes, push the child seat into the vehicle seat to get the belt nice and snug, but it's not friction fitted like some other seatbelt fit car seats, so there is no need to use your entire body weight. As long as all the slack if out of the belt, the seat itself doesn't move (that would be forward anchorage) and the lock off is tight, it's good. And then you want the harness as close to his shoulders as possible, if he is between two positions, choose the higher one. No higher than 2cm above the shoulders. When you strap him in, try to make sure that he sits upright with his bum to the back of the seat, and his shoulders resting on the backrest. If they lean forwards, they can sort of get stuck in a hunched over position with the belt pinning their shoulders away from the backrest, which means you'll have slack in the harness plus it will be uncomfortable for him.
Other than this some Peugeot cars have isofix + top tether in the front seat, maybe Citroen do as well?? It is a long shot but worth checking. If you have isofix + top tether, you could use Joie Bold or Joie i-Irvana Max, both of which are up to 25kg.
For seatbelt fit up to 25kg FF, Maxi Cosi Beryl is the best bet as said, although be aware the shoulder height is more restricted - it's about the same max height as Joie 360 Spin. It looks like it can still be purchased but I don't know how reputable the sites are that have it in stock. It fits using a base which separates from the seat itself, the base can be isofix or seatbelt fitted, and must be seatbelt fitted when the child is over 18kg.
If you are going to try RF, it doesn't make sense to have him in the front so in that case, I would remove the rear seat headrest, try to block view from side windows and give as clear a view out the rear windscreen as possible.