@DCNehs
OK...
Here is how I suggest that your proceed.
I would, as you have suggested, use a separate column to record when the person or Group column has been set. You can use a condition that checks for your date column being null - if it isn't then don't do anything.
Next, I would also add another text column and set it to some value if the multi-line text column is not empty - use the same concept as you did for the date column.
Use a Schedule trigger that runs once a day.
Use a SharePoint Get items action with a Filter query so that only items where the Date column is at least 10 days from today and where the text column you created is null. The reason I suggested createing a separate text column is that you can't filter on a multi-line text column.
Then, you'll need to have a condition that checks for 10 day intervals. That will get messy., You'll need to find the difference between today and the date column and divide by 10 and check the result.
Probably more complicate that it should be...
