Is feeling intense fear over money and the future, reducing sadaqah (voluntary charity) while maintaining zakat (obligatory charity), and being influenced by the results of istikhara prayer that lean towards not spending money, considered stinginess? And does this carefulness with money affect the validity of istikhara prayer?

The best approach is moderation in spending, avoiding both extravagance and stinginess. As Al-Ghazali said, "Money was created for the wisdom of fulfilling the needs of creation, so it must be preserved where preservation is due, and expended where expenditure is due. Withholding where expenditure is due is stinginess, and expending where withholding is due is extravagance." The middle ground is praiseworthy, which is generosity and magnanimity. As stated in the Holy Quran: "And [they are] those who, when they spend, do not spend excessively and do not spend restrictively but are always [observing] a just mean between these [extremes]." Stinginess is a blameworthy ailment, as described by the Prophet, peace be upon him, just as extravagance is blameworthy if it exceeds limits. Good wealth is for the righteous person. Discomfort after Istikhara is not sufficient reason to abandon an action; rather, one must proceed with it and strive against the self to give and bestow in order to uproot stinginess and avarice.