You probably can automate the conversions you have described to some extent with AppleScript but sorry that I don't know how.
Until you figure this or another automated solution you might use the following keyboard shortcuts in the given order to select multiple rows and copy & paste them in-place as values. These are likely to reduce the time you spend on such conversions if you are not already using them.
- Go to the first cell in the last row you wish to convert to values,
- Press Shift ⇧+Command ⌘+Right Arrow → to select all of the row,
- Select as many rows as you like by pressing Shift ⇧+Up Arrow ↑ ,
- When finished, copy all the rows by pressing Command ⌘+C,
- Paste all the rows in-place as values by pressing Shift ⇧+Command ⌘+V.