Glad you got good results!

Part of the obstruction of visibility in a pond comes from electrically-charged clay particles and part comes from blooms of various living organisms.

The alum only works on the clay particles.

Do you have a swan as a permanent resident on your pond? Do you feed the koi, or do they stir up your bottom sediments when they get hungry?

Swan (and goose) poop is straight up fertilizer for the blooms in your pond. Extra koi food that they miss and lands on the bottom also becomes fertilizer. The poop from well-fed koi is also fertilizer. The more "sterile" you keep you pond, then the better visibility you will have. Obviously, you will have to choose your trade-offs, because you clearly do not want a sterile pond.

If you do decide to keep 100% of the existing animals that are "fertilizing" your pond, then you certainly want to eliminate any other sources of fertilization. Make sure no run off from fertilized yards reaches your pond, or from fertilized ag fields. Also, no fertilizer from stock animal wastes. Finally, it even helps to keep grass clippings, leaves, pine needles, etc. out of your pond. Those all have a slight fertilizer effect. Further, the organic material that is NOT broken down goes on to create organic muck on the bottom of your pond!

One other option to reduce your blooms might be some pond dyes. They work by reducing the sunlight in the water column available to the photosynthesizing organisms. There might be a possible delicate balance where you reduce the sunlight enough to affect the algae yet still get enough visible light through for your eyes such that you can get even deeper secchi disk readings. However, that may be quite difficult, and there will certainly be a color tint to your water. If there are some tints that you think would make your pond MORE attractive, then it might be worth you experimenting.

As to more alum treatments, I think you need to perform some periodic "jar tests". When your jar tests indicate the visibility problem is due to suspended clay, then you can consider another alum treatment.

I would also invest in a good water testing kit. I think you should have results in your file showing your pond chemistry when it is healthy and presumably can easily accept another alum treatment.

There are several good threads on jar tests and testing kits in the archives.

Good luck on your beautiful, clear pond! I think 30" visibility is pretty good, but you certainly have several options to go for more.