Agreed.
I know. I was just sharing my thoughts. You're probably right. It's likely more difficult to fix an amendment problem mid grow. I did it due to stubborn curiosity.
I already wrote most of my thoughts so far. Basically this: Figure out what was depleted in the soil and then amend it back in. The first problem I saw with reused soil was magnesium deficiency, so I added some to the plant's diet. It helped but there was still a problem. The next deficiency was harder to diagnose. I decided it was phosphorous deficiency, so I foliar fed some FF bloom nutrients. The plants liked that a lot, so I top dressed the soil with a mixture of GGAP & GGPB. I over did it a bit and got some leaf tip burn, but the plants otherwise responded very well. That led me to theorize that the most likely depletion would be that which the plant used most during flowering, which, of course, was the last use of the soil. So, I have test clones in a soil mix with both GGAP and GGPB, instead of the GGAP only that I'd usually use. They seem to be growing well in that mix, but they stretched a lot because they're in a tent and light that I pieced together for the test. I'll have a better tent and light available for them in a week or so.
Edit/Added: I just recently read that slow growth is a symptom of phosphorous deficiency, and that's one of the problems that have been described. That supports the phosphorous deficiency theory we've been discussing.