Soymilk for drinking vs. soymilk for tofu making Many recipes for tofu start from soymilk, many others start from scratch, but most of either kind share a whole variety of approaches, i.e. the order of the soaking, boiling and blending operations, the duration of the soak, the length of the milk cooking/simmering, whether the milk should be strained before simmering or the other way around. Recipes for drinking soymilk vary on a similar fashion, apart from the fact that they aim to remove the so-called "beany flavor" that seems to be haunting homemakers.

Assuming the invariance of the final product in terms of flavor, firmness, coagulation etc. there are some choices that result in consistent operational advantages, such as the boil-and-blend method which leaves out the discomforts of the soak, or the option of straining the milk before the simmer so that all you have to do to get to the coagulant insertion is wait for it to come down to temperature.

My question now is: can we make such an assumption? Some other sources suggest for example that a soaking time longer than eight hours inhibits the soymilk's ability to coagulate properly, but others point out that it would yield a better drink. Same goes for simmering time, let alone all other variables and orderings I briefly listed above.

I tried making tofu twice using a combo lemon juice and salt as a coagulant, but the results were ricotta-like and silken tofu, while I strive for firm to extra-firm. I mostly blame it on my previous lack of a cooking thermometer which I just purchased specifically (together with some nigari crystals) for this purpose, so I thought I should ask if there are other parameters I might be leaving out of the investigation.

TL; DR: Just read the stuff in bold.
Your title, your problem and your question (btw question is general but complicated) while on the same subject matter are not the same.

In regards to your quest for firm and extra-firm tofu, the OP doesn't state much about the process after the coagulation.  Which similarly to cheese making is the (arguably) the most crucial factor in the final texture.  firm and extra-firm tofu is pressed with heavier weights, and for longer periods of time.

The choice of coagulent (nikari, gypsum, acid, etc), temperature of the milk and the mixing method can also affect the texture of the final product.