Do not fit isofix yourself it’s not a safe thing to do. 36kg/125cm seats that don’t need isofix: Axkid minikid 2/4, Axkid movekid, Britax Safeway/maxsafe, BeSafe stretch You didn’t give a heigh/weight so I can’t really recommend them but Joie make seats that are belt fitted but they are outgrown sooner
@Kierrah hi thank you I’ll look into those. I don’t know how tall she is but she only weighs 9kg
You can still use the seat without an isofix. The seat should have where the seatbelt can secure it.
Most tethered seats don't need isofix are there any tether points in front of the seat you can use, if so I'd recommend the anxikid minikid 4 or movekid for extended rear facing. If not the joie everstage is pretty good I've been told but not tried it personally that just fits with the car seat