Maybe it’s just my American perspective, but I’m a bit uncomfortable with this whole idea, voluntary or not.
The state who collects for X has the power to coerce X.
It's obviously suboptimal, but it is standard European practice for all majority churches, Catholic or Protestant.
The state who collects for X has the power to coerce X.
Which is precisely the scenario that is playing out right now.
Under a Popular government, the Church could be comfortable that it would have the funds to run hospitals, schools and shelters.
When the Socialists were elected, they went to war with the Church.
The Church's best strategy would be to begin a 10 year plan to wean itself off government support while fighting for its rights every step of the way.
And if the Spanish government is going to stop supporting the Church, it should cut taxes so the people of Spain have sufficient disposable income to make charitable donations.