There's some validity to what you've written, but some of it is patronising and seems to attribute positions to me that I didn't convey, or indeed conveyed an opposing position.
Note that I specifically wrote "Of course one should work with a health professional when considering taking dietary supplements".
That's what I do - I have multiple practitioners, and have my levels tested periodically (blood testing is of limited value with magnesium, whilst hair testing can give a better indication, though it has its own pitfalls regarding the testing procedure and the interpretation of the data, which need to be well understood by the practitioner).
You've also focused on the risk of "megadosing", which I don't practice or advocate for any nutrient.
Indeed, the real point of my comment is that supplementing any single nutrient by itself is likely to have consequences that you'll be unaware of without really doing your research. This article focuses on the risks of "megadosing" vitamin D. I'm not sure how you interpreted my pointing out that vitamin D supplementation can lead to magnesium deficiency as a recommendation to "megadose" magnesium. I've certainly never taken magnesium in any quantity that gets close to toxicity and I'd never advocate anyone else do that.
(Incidentally, I've felt before that Gwern's approach of experimenting with supplementation of a single nutrient and looking for a single change is of limited value, as it doesn't take into account the complexity effects of that supplementation. His list of self-experiments includes a magnesium/sleep trial and a vitamin D/sleep trial, with negative results in both cases. But for the reasons discussed above, a trial involving both magnesium and vit D, and perhaps other nutrients as well would likely yield very different results, and probably ones that are far more useful).
Your comments about the efficacy/cost of higher bioavailability supplements are worthy of consideration. Though it seems contradictory to warn against "megadosing" in one paragraph but then recommending increasing the dose in another.
The particular benefit I'm interested in is passing the blood brain barrier, as the biggest issues for me are mental - e.g., anxiety and memory, for which magnesium l-threonate is said to be particularly beneficial. I would expect that increasing the dosage of a different form, which doesn't cross the BBB so effectively, would potentially lead to an excess elsewhere in the body.
To be clear, I'm not primarily focusing on magnesium; the main target of my approach to healing is currently the thyroid, and I'm cautiously taking tyrosine and iodine for that, as well as working on gut/microbiome health to improve uptake so that ultimately all nutrients can be attained from the diet so and no supplementation is needed, which is ideal as you point out. But I'm not there yet; several years of testing and work has confirmed that.