Naturally Native has quite a list of plants for erosion control. Many of the Coprosmas, Leptospermums, and Phormiums look quite promising. As for Agapanthus not succumbing to glyphosphate, that's not true in the United States.

If it grows and is available where you are, go for Cotoneaster procumbens 'Queen of Carpets' - height up to 6/8 inches, spread indefinite, slow to grow at first, roots as it grows, flowers in spring, berries later. Very popular with bees.

Otherwise, Hebe youngii (previously called Hebe 'Carl Teschner') - you'd need a few, height 6-8 inches, they only spread to about 3 feet eventually, but they also tend to root at the tips as they grow. Also Hebe pinguifolia 'pagei', and Hebe Wingletye, similar height and spread - all native to NZ.

If you want a little bit of height here and there, and a break in the solid ground cover, add Yucca flaccida 'Golden Sword' - max height 2-3 feet, evergreen, highly drought resistant. They might look a bit weird on a vertical slope, but no weirder than agapanthus might.

All these plants need some sun, so if the bank is in total shade all the time, they won't work - but I'm assuming, if you're growing agapanthus currently, there must be some sun exposure.