To flush LECA medium with tap water or reverse osmosis water?

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Here in AZ with very mineralized water of high pH, there are several ways around these problems.

pH of 8.5-9 just doesn't work for most orchids. Not many nutrients will be absorbed. Testing with an aquarium pH kit for high pH, you can try adding a tablespoonful of regular white vinegar to a gallon of your water, checking the pH, then repeating until you have something around 7. From that point forward add that amount of vinegar to each gallon of tap water you use.

You didn't describe what kind of self-watering system you use. Some of them suck up water from below, then evaporate it from the surface. Others (Ray's semi-hydroponic system, for example) provide water from above, but still evaporate mostly from the top. The problem is mineral buildup on the medium surface. This can be minimized by not allowing the medium to go completely dry, or by watering from above, frequently. With Ray's S/H method one can water from above frequently - daily or more would not be a problem. Also, with Ray's system, one should completely fill the entire container at each watering, displacing all the air.

If the medium never dries completely, and you flush through from the top frequently, you will have much less trouble with salt buildup in media.

Most desert cacti, by the way, do better with high-mineral, high-pH water. They are much less susceptible to various fungi this way. Most house plants do not like high pH and high mineral content. This is why so many develop brown leaf tips - the water evaporates and salts concentrate there, killing the cells. The dead zone moves back towards the leaf stalk as the distal parts of the leaf die successively due to high salts in the water.

For a couple of months I've been using my 600-1200 ppm water with a spray system for my orchids for most waterings. I will hand water with R/O about once a month. It's been working OK so far.

I forgot to add at first that using R/O, rain or distilled water allows relatively easy control of most water factors. A type of fertilizer called MSU (for Michigan State University) is formulated two ways: One for well/tap water, one for pure water. Each formulation contains various minerals that yield a reasonable pH when used with the expected type of water.

I collect and save rain. Look into that - during a summer monsoon storm I can easily collect 400 gallons of rain in an hour. I use 12, 33-gallon lidded plastic trash barrels to store water in my sunroom. I use a sump pump to move it from the stock tank outside under my rainspouts into the sunroom.