Almost impossible to answer really, as it depends on so many factors (as others have said).
Your car is now 6 years old and is comparable to the sort of cars that can be sourced through Motability at a cost of around £12,000 sacrificed benefits plus whatever the advance payment is. Only you can determine whether that is good value for money for you. Motability provides ‘worry free motoring’, albeit at a cost. Servicing, tyres and insurance are all included. Road tax is also included but you can now get road tax exemption on a private car due to your wife’s entitlement. Your car will be out of warranty whereas a new one will be covered (whether you buy through Motability or privately). The same with breakdown cover.
Motability are trying to push everyone towards EVs and, as a result, the choice of conventional petrol or diesel cars is now pretty dire, especially if you want a decent sized car. If an EV is attractive to you, Motability is an excellent route to ownership, as you won’t be exposed to the depreciation issues that are currently affecting them. If you don’t want an EV options are getting more and more limited, but as you are happy with a Kia there is still a lot of choice from them and similar brands (Hyundai etc).
Having been with Motability for many years we were thinking of leaving, as the sort of cars that we would buy in the private market aren’t on the scheme. However, another area where Motability is excellent is adaptations and my wife now really needs a hoist to get in and out of the car, which has tipped us back towards Motability as I wouldn’t really want to fit one to a privately owned car. Lots for you to ponder over!