It is interesting how hard is to decouple legal from moral.
"Elsewhere in EU" point of view - how it would be modelled here.
Deprivation of assets and inheritance tax avoidance aren't in play from local point of view.
Given the property ownership was handed owner, the will isn't relevant any more either.
You received conditional gift. Condition is life long right of stay for your parents and respectful behaviour to them. The condition is marked on the deeds, so if you would sell/lost the house their right od stay remains (and therefore lowers selling value).
As owner, you are responsible for maintainance (both paying and performing), tax and other costs of ownership. Depending on the conditions of the gift responsibility for utilities bill is probably theirs.
The idea is, that they are giving property ownership over early in exchange for you taking care of the property as of your own. You are getting rather significant value, so it makes sense to expect some small percentage given back for your parents benefit. The alternative would be for them to sell it and use the money for their own benefit, or apply for reverse mortgage, so you are still better off.
If the property doesn't work for your dad anymore, lawyer can help you with a new agreement where the right on the first property is lifted in exchange for something else (money, right on another property, but it must be beneficial for him).
It comes from our law, but I see it as reasonable and moral.
The early inheritance comes with strings attached and it is there to protect your parent's home.
In your case - not being able to care about garden isn't a reason to move him, you can cover that as an owner.
If you want to move him, there must be agreement. Agreement must make sense to both. In general, half of the gift value comes from him, so that is the part he should co-decide.
You are probably in different country, as you would know if you had right of stay registered, I'm just offering "how it would work" in another framework.