App Selly can do exactly what you want.
In bundle offer setting, you can create many product groups and set different discounts for different groups:
Optionally a bundle widget can be displayed on product page:
Regarding to your example, you will need to create 4 bundles:
Bundle 1 contains 2 groups: group 1: no discounts, group 2: 25% discount
Bundle 2 contains 3 groups: group 1: no discounts, group 2: 25% discount, group 3: 30% discount
Bundle 3 contains 4 groups: group 1: no discounts, group 2: 25% discount, group 3: 30% discount, group 4: 40% discount
Bundle 4 contains 5 groups: group 1: no discounts, group 2: 25% discount, group 3: 30% discount, group 4: 40% discount, group 5: 50% discount
Discounts will apply to the cheapest items automatically. The cycle starts again when there are more than 5 items.
The app also has other features such as gift, motivator:
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