Simple way... I can propose you how user can do it in just one step, but I don't think it is suitable to current mental model.

I think that lines between columns or rows can be used to add a columns and rows. Click twice on the line between rows to add a row. The advantage there is that user already is pointing where the row supposed to be, so it is all done in one action. Sadly there is no place for a signifier to notice user that this can be done that way. You can only notify it by some note or tutorial. This also mean you still need implement also a button for adding rows and columns.

You can wider lines on hover, to notify user that there is some action possible, but you can't put clear signifier that is saying what action. Also I'm afraid how users will react to this. I'm imaging that it can be irritating, when all this lines are reacting to hovering. Another possible solution is to change pointer into '+' when it is hovering lines.

Clearly, this solution need a research.

Simple way... I can propose you how user can do it in just one step, but I don't think it is suitable to current mental model.

I think that lines between columns or rows can be used to add a columns and rows. Click twice on the line between rows to add a row. The advantage there is that user already is pointing where the row supposed to be, so it is all done in one action. Sadly there is no place for a signifier to notice user that this can be done that way. You can only notify it by some note or tutorial. This also mean you still need implement also a button for adding rows and columns.

You can wider lines on hover, to notify user that there is some action possible, but you can't put clear signifier that is saying what action. Also I'm afraid how users will react to this. I'm imaging that it can be irritating, when all this lines are reacting to hovering. Another possible solution is to change pointer into '+' when it is hovering lines.

Clearly, this solution need a research.