Are the spouses sinful if they stop procreating through 'azl (coitus interruptus) or by regulating times of intercourse, not out of fear concerning provision, but because they do not desire more children? And what is the ruling regarding those who marry with the intention of chastity (tahsin al-farj) without the intention of procreation?

The spouses are not sinful for not having children if it does not entail a Shari'ah prohibition, such as using something that completely prevents conception. It is preferable not to refrain from procreation due to the benefits it holds for the parents and the Ummah. As for marriage with the intention of chastity without procreation, there is no harm in it. However, the spouses may thereby be deprived of much good, for a person's intention is more impactful than their action.