There is no thing such as "modern mortar" as far as I know. Mortars can be adjusted for strength so that they are not stronger than the block or stone. You are correct that a mortar that is too hard will create issues for the wall in other ways.
Have you found a mason to work with? There are ways to test the wall and mortar and properly match them.
I am not a mason but one of these guys on here should know and point you in the right direction.
Have you found a mason to work with? There are ways to test the wall and mortar and properly match them.
I am not a mason but one of these guys on here should know and point you in the right direction.
