The context: I have 24 years left on a mortgage I refinanced down to 3% during lockdown. Fifth-Third bank now gives me an option to switch to biweekly payments whereas they couldn't before. They do assure the extra 'full payment' goes to principal only, and it would shave 3 years off my loan with an interest savings of $6,023.

However, running the compound interest math on that extra yearly payment of $943 in an index fund...By the 21st year mark, it's projected to be $52,29. Am I doing my math correctly, or leaving out other beneficial factors?