Magento2 change Category-Product position to string

I would like to change the category-product position to be a string. Because I have an exploded view for spare parts and they are numbered with strings (1a, 1c, 605,…). The customer should see the position to select the right parts based von the image.

I will update this only with the API:

In Magento 1.x I changed the database field to string and something in the magento core Code Category Model from (int) to (string).

But I know, that this is not the correct way to do this. Creating an extra plugin to extend the database and modify the API seems a bit “too much” in this case. Where can I start?