How to set custom product quantity increments in PrestaShop to x10 and to round quantity to nearest 10

Question

platform: ps. 1.7.7.5

I sell a 10pc./pack product. I have different price points for each quantity (100, 200, 300... 5000). Currently I use variations to set up quantity options with different prices per pc., but it makes accounting complicated and gives less options for customers (for example customer might want 150pcs).

I want customers to be able to:

1. Click "+" to add quantity increment of 10pcs., every "+" click will add 10 more pieces of product.
2. Write any quantity in the field and that quantity will be rounded up to nearest 10 (e.g. customer writes quantity 455pcs. - it rounds up to 460pcs.) It should work on product page and checkout too.

I assume this changes have to be made via core file editing, maybe someone knows what file I should edit? If possible I would like to avoid using additional modules. Thanks in advance.

Tried to look for similar questions, didn't succeed or found one about ps. 1.6, but it doesn't apply.