The thing about ponds is, you will not have a "natural" pond where there is mud and plants and fish and algae and so on. A natural pond has a very small fish load per volume of water. You will want more fish, so you can see them once in awhile, and more plants, so the pond is pretty. Both fish and plants create a lot of debris, dead plant matter and fish poop. You have to fertilize the plants monthly to keep them blooming robustly. You have to feed the fish to keep them coming to the surface so you can see them. Both accelerate the work you have to do.
Then, once or twice a year you need to "muck out the pond" which is get to the bottom somehow and remove the accumulated poop and plant debris that are making a sewer on the bottom. In winter the gasses from this debris can accumulate and kill fish and the wastes there can breed nasty bacteria. If you think treating algae in a pool is pricey, just wait until you are treating for parasites or bacterial infections on a per gallon basis. Rock bottom may disguise this trash for a few years but eventually it must be cleaned and it is much worse then with rocks in the way.
And, goldfish and koi both eat plants or roots. So, if you want flowers you have to protect the plants from the fish. Without the fish you get mosquitoes. With out the plants you can still do the fish, but it just isn't as pretty. You can only see the fish when you are close, nothing to see from the patio or the house if there is not plants.
Been there, done that, it was fun, beautiful, and a lot of work on occasion.