If the original foam is deteriorating, I would remove it and replace with backer rod. The 3/4" is fine as you can work it into the joint for a tight fit.
To make a consistent depth, sweep play sand into the joint. Use a jointing tool (narrow mason tool-about 1/2") to work in and create a solid base about a 1/2" deep. Then apply the Deck-O-Seal.

You will use less material and prevent sags and have a consistent thickness.

Be patient and careful when applying with the squeeze bottle. If you get drips on the coping or patio, it's difficult to remove.