This feel like a customization is needed. As standard system checks discounts only on line level and it is not possible to group items.
I would say You need a customization - to create
Item Groups (table and page, enough with just 2 fields - Code and Description)
Add Item group field in Item card.
In Item discounts (Sales/Purchase) (new version would be Price List Line (table 7001)) Add option in "Product Type" drop down list also for Item group and provide the minimum quantity 100 and discount 5%, and another line for Min=200, Discount 10%.
Then on Sales Order - on Release button add code for re-calculating the discounts for Item Groups and adjust all lines.
I am pretty sure there are at least a few extensions that can achieve this, but I haven't worked with such, so cannot advise any.
