Soil amendment option - SD garden beds

[SDDune]

Been lurking for a while and finally joined. Nice forum, great advice from members!

Our xeric front yard/garden was supposed to be amended with 50% pumice in all beds when it was installed in April. It was not. Long story for another day.

Our soil is a sand/clay/loam mix on the bay/coast. It's not the undrainable clay of inland SD, but it does hold water more than rocky loam. It never puddles, even in low spots, and holes do drain in under an hour. We were watering according to the plan for establishing plants in the amended soil, in a typical SD May/June, and lost 50% of the echeverias and all the diplacus by the time I realized what was happening. My bad. But the agaves, aloes, and aeonium seem fine.

It's not an option to start over and re-dig & amend the beds around the hardscape. I've saved pups and some rosettes on the echeverias; they're rooting in the shade out back.

QUESTIONS:

Will this plan work okay, if not be ideal? My current plan is:

I'm drilling drainage holes around each plant that seems established and doing fine (agaves, aloes, aeoniums). They are 3' deep holes, 2" wide, filled with pumice and drilled to angle below the roots/downslope of each established plant.

For the new/replacement plants, I'm planning this:
1. dig planting holes 2x the depth & width of each plant, then
2. drill a 2' to 3' deep hole 2" wide in the bottom of each planting hole,
3. fill that drilled hole with pumice, then
4. fill the planting hole itself with an amended 50% pumice/50% soil mix

Would that provide enough drainage for most xeric plants?

My goal is a solution that I can implement myself nights/weekends.

Thanks!