Sum for a Calculated Column

I would suggest on of the three options below:

1. Instead of a Calculated column, use a Currency column and use Power Automate flow to manage updating the total for each row. Since an actual value is added to the Total column, it will show the Sum correctly in the footer. The trick here would be when to run the flow (only trigger when one of the month values has been updated) to ensure you don't end up with an infinite loop.
2. What I would likely do myself is just use an Excel Table to manage everything which will be able to show all the Totals, Grand Totals, etc. using formulas. You can also lock down certain cells such as Total so the formula can't be overwritten. Is there a reason you have it as a SharePoint list?
3. Use a Table in Dataverse (and maybe a Model Driven App) that will allow you to have calculated totals for each row, and grand total across rows.