Id say no one has jumped to answer this one because pH, acids and buffering has been done to death.
Be careful taking advice from pond or aquarium shops due to they are not raising fish for human consumption nor are they trying to grow veg. It is true fish prefer higher pH ranges, as does the nitrification process but its well documented that most of the fish we are growing can happily live in broader ranges so we tend to adjust pH to more so suit plant growth.
Heres a helpful chart showing nutrient availability at certain pH ranges.
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Optimal pH isn't really cut and dry. As mentioned, a higher pH is preferable for bacteria growth so that is helpful when cycling and stabilising the system, most systems seem to start out at higher pH levels anyway so recommendations from us early on is to leave it be and let the system develop and in most cases due to the acidic nature of the nitrification process it comes down in time anyways. Bouncing pH levels will cause more harm than good.
When your focus switches to plants this is usually when you want to control your pH into a range best suited for optimal nutrient uptake. There are a bunch of products you can use to do this. As you said, citric acids are no good due to their antibacterial properties, phosphoric acid will work but there are better options by the use of alternating buffers and treating top up water.