It's not clear to me what exactly how your interest is calculated

If the interest rate depends only on the balance and applies to the whole balance I would use a VLOOKUP function to get the rate. You set up a small 2-column table of balances and rates (usually on another sheet) and use an 'approximate match' to find the rate corresponding to the balance. LOOKUP functions are hard to get the hang of but really useful in lots of circumstances. If the rates change, or tiers are added, the interest formula does not change.

If different rates apply to portions of the balance I would add more columms, each with a calculated portion, and a single formula to calculate and sum the interest on each.

You could use both methods together - extra columns with tiers and rates in a table. Depends how often these change.

Personally I would avoid nested IFs. Too Easy to make mistakes.

HTH