Default variation from of multiple variations on product page

I have around 2000 products with each product having 50 variations (sizes), our existing ecom store we just selected a default on each product page for the variation, we use the warehouse theme and the default behaviour seems to be the first variation in the list.

we currently have something similar like this

Variation 1 - 1 metre

Variation 2 - 2 metre

Variation 3 - 3 metre

Variation 4 - 4 metre

Variation 5 - 5 metre

The variations are not always the same for each product but we need to select out choice for the product which for this example would be variation 3 to be displayed when the customer opens the product page, I have searched the forum and found links to various coding but it seems to be overly complicated, if we have to pay for an app then so be it but am I missing some simple way to make this happen?

thanks in advance