Definitely, I would try acid cleaning. I am surprised you went this long without an acid clean. That is usually the first thing I do when I detect a decrease in cell output. Manual cleaning only gets the deposits you can see which is basically the ends of the cell. The internals are not easily cleaned manually.

Also, I clean my cell with acid several times during the season and the cell is over 7 years old and still going strong. I don't use full strength (1:4) and just long enough until all the fizzing stops.