You could follow the same pattern that you already have established by removing the previously calculated values from the total, leaving the days.
However, I would probably approach the problem like this..
1. Have a column that calculates the years difference between the dates
2. Another interim column that calculates the start date plus the number of years in 1.
3. Another that calculates the difference in days
between the interim column in 2 and the EndDate. Divide that value by 30.5 and convert to an inteer to get the whole months
4. Another column to Add that number of months to the interim date
5. Finally calculate the number of days between the interim date in 4 to the End Date
You can hide the interim columns from the final report, but breaking them out like that makes it a whole lot easier to spot mistakes than your highly complicated formuala.
Alternatively, I did a similar example in SQL on this post How to calculate month and days between two dates
] which you could convert to a Function instead.
Footnote: the reason for the division by 30.5 is because that is the average number of days in a month and is surprisingly accurate when used over a period which includes or does not include leap years. You could be pedantic and use 30.4 instead. Just remember to truncate to an integer.
Edit: Just in case here is a link on how to hide items in Report Builder Hide an Item (Report Builder) - SQL Server Reporting Services (SSRS) | Microsoft Docs