What should I do with land I bought with an interest-based loan while continuing to pay the installments, given that I now have an opportunity to sell it for a price much higher than the loan amount? Should I donate the excess amount to charity or benefit from it?

Your borrowing from a usurious bank with interest is reprehensible and forbidden. Since you have repented, we hope that Allah accepts your repentance and forgives you. You must rid yourself of theaming forbidden contract. If it is possible to repay the loan with other money, then do so. If that is not possible except by selling the land, then sell it and repay the loan from its price. Whatever remains is lawful for you to dispose of as you wish.