As pointed out by @JuannStrauss, a spare tyre that is stored inboard of the vehicle will degrade more slowly over time than one of the regular wheels since it is not exposed to:
- normal wear & tear from vehicle motion
- other physical aggression (hitting a curb)
- winter salt
- heating in summer
- UV light
All of these dry out the tyre composants and degrade their performance. However, even in their absence tyres still degrade slowly over time. Many vehicle owners also forget to keep the spare tyre inflated to its correct operating pressure, so some deformation can occur over time.
This means an old tyre will be more exposed to over-heating when eventually run, leading perhaps to a blow-out. If you do need to use it, run at moderate speed over as short a distance as possible (no leaving the blown tyre to repair next week, for example).
As a matter of personal opinion, I would probably risk it, but exercising much care - and not with a tyre going on 10 years as you are about to do. However, you should also be aware of the possible legal repercussions if you should be involved in an accident. The other side's legal representatives would no doubt jump on the occasion to try shift the fault onto you, and expired tyres are an objective argument that is hard to counter.
Also as pointed out by @JuannStrauss, having five tyres of identical size allows you to rotate between them at each regular inspection. However, in your vehicle the wheel receptacle is too shallow for a regular tyre, and you may deform the lower boot surface (carpet). If you do go down this path, I would simply pack the spare tyre in a garbage bag, and lay it flat on top of the carpet, not in the receptacle. You will loose some boot space, but you have plenty of that in your vehicle.
Hope this helps.