Another tool to use is this:
https://www.mortgagecalculator.org/additional-payment-calculator/
A way you can do a comparison for a 30 vs. 15, resetting at 30, etc. is you can add in additional one-time or monthly payments to get the monthly payment the same for each loan, and it will show you when it will be paid off.
For example, you can put in your specific loan details for your current mortgage, and it will tell you when it will be paid off. You could then put in a fresh 30 year refinance. Obviously your payment will be less because the rate is lower and you are starting over with the term. You can then add in the extra monthly payment amount to get the monthly payments on par and compare payoff date and total cost of the loan.