In Odoo, you can exclude certain packaging from the website by setting the "Website Visible" field to "No" for those packagings. This can be done on the Packaging form under the Inventory module.

To offer bulk discounts for larger size packaging, you can create a pricing rule under the Sales module. To do this, go to the Sales > Configuration > Sales > Pricelists and click on "Create" to create a new pricelist. Then, go to the "Pricelist Items" tab and click on "Create" to add a new pricing rule. In the "Product" field, select the product for which you want to offer bulk discounts. Then, specify the conditions for the bulk discount in the "Apply On" and "Based On" fields. For example, if you want to offer a discount for customers who buy more than 100 meters of cable, you can set the "Apply On" field to "Product" and the "Based On" field to "Quantity" and specify the minimum quantity in the "Minimum Quantity" field. Finally, specify the discount in the "Discount" field and click on "Save" to apply the pricing rule.

It is up to you whether to use variations or packaging to offer bulk discounts. Using variations may be easier to set up if you already have variations for different cable types, but using packaging may be more intuitive for customers if the packaging is the way the product is commonly sold.

Product packagings can't be selected from ecommerce, it is possible on sales app. Also, if you were to use these as variants, they would be selectable but it wouldn't update quantity. It would only update desription and price (if extra prices is configured for variant). For example someone wants to buy 2 rolls, they would be selecting rolls as variant and qty as 2, on confirmation inventory would show as 2 mtrs and not 200 mtrs

If the various options are tracked individually in your inventory then you may want to create these as different products and define UoM as units or length (you can also create Rolls and Drum as UoMs).. ie cables from Rolls & drums aren't interchanged.

You can exclude a product on website by not publishing it.