That's how the Joie Every Stage R129 is. It's different from the older model Every Stage which had a forward facing harness mode as well.
Rear facing R129 models with better leg room would be the ones that fit with tethers so they can be moved back or forwards in the car. These are also the longest lasting rear facing seats on the market:
Besafe Stretch
Axkid Minikid 3
Klippan Opti 129
(Also Avionaut Sky because somebody will say I've forgotten it, but this has less leg room)
Or there are a couple of rear facing R129 models with isofix which fit on rails so can have more/less leg room:
Joie i-Prodigi
Axkid One / One2
Or you have the Besafe izi Twist / Turn which is an R129 spin seat which has more leg space than most other spin seats. The Twist is rear facing only whereas the Turn can also be forward facing. There are plenty of other spin seats with a forward facing option which are R129 compliant but they don't tend to have much leg room.
Joie Steadi R129 might be worth a look as this is belt fitted rear/forward facing. Not much leg room rear facing though and if you're going to use forward facing, I'd just get an isofix seat.
Or lastly a list of R129 forward facing seats which go from harness to booster:
Britax Advansafix i-size
Silver Cross Balance i-size
Recaro Tian Elite
Maxi Cosi Titan Pro i-size
And one wild card: Cybex Anoris T, which is R129 but a totally different type of car seat with an impact shield and airbags. Suitable up to 115cm.
I'm curious as to why the R129 requirement is a must? You don't have to say (as you can see, you've plenty of choice!) I'm just wondering. There are a lot of common misunderstandings about this regulation so it might be that we can expand the search if the reason is one of these - may be especially helpful if you need something seatbelt fit and/or on a budget.