Is one who delays repaying a debt past its due date, while experiencing severe financial hardship, considered sinful?

You have done well with your good intention to help others. Although borrowing money for that purpose was not necessary, you did it out of benevolence, and a person is not to be blamed for doing good.

It is obligatory upon creditors to grant you respite if you are in hardship, for Allah the Almighty says: (And if there is one in hardship, then grant him respite until a time of ease.)

Allah the Almighty has promised that whoever takes people's money intending to repay it, He will repay it on his behalf, as the Prophet, peace and blessings be upon him, said: (Whoever takes people's money intending to repay it, Allah will repay it on his behalf.)

As for the suggestion to demand repayment from those you helped, it is incorrect because it is a form of retracting a charity, and demanding repayment from them is a harm that nullifies the charity. Allah the Almighty says: (O you who have believed, do not invalidate your charities with reminders of your generosity or injury.)