Not all cassava is good for making garri. Most cassava that have higher content of starch like 419 may not be suitable for garri making because of high starch content.
However, if you want to use 419 for garri you will need to harvest it during raining season because most of its starch would have been concentrated at the leaves and for forming growing branches.
If you harvested it for garri during dry season as root crop, at dry season its starch would have been concentrated at the root so the decision is now yours.
Have a plan or target for your cassava. By so doing you will be able to determine varieties to plant or when to harvest new plant and harvest your cassava.