What does say in the ‘Current lease’ section in your online account ao how lng you can extend. It may say different to what you were told!
If it only shows 6 months, extend for that time. Near the end of 6 months tell Motability, you regret doing so but, you will have to leave the scheme as there is nothing suitable. Its highly unlikely Motability will want to loose you as a customer after many years and likey allow you to have another extension.
The biggest issue as I see with this, is the longer you keep extending your current lease, the less ICE cars will be available.