Firstly, I honestly appreciate your nice informative/applicable query whose at least benefit can be this that they should be aware that whatever they do (will do) is relevant the will of Allah …
Secondly, in regards to your sensitive inquiry, based on my research/perception, such using of this verb “… do not say of anything … Except if Allah wills”
doesn’t indicate the Wojub (being Wajib/mandatory) of saying this phrase (ان شاءالله) by tongue, whereas its main goal is showing that we should not being confident of doing our act for the future (and even currently) without noting that performing everything is completely relevant to the will of Allah, otherwise the action/errand won’t be complete/done.
In fact, it wants to teach us regarding the truth (hidden) aspect of the issue (and being aware of that), not being wajib in using that phrase orally. And actually it emphasizes on the politness of monotheism (as a fact that we always ought to regard that.
Eventually, let me fulfill my response by giving an example (which can be related to our discussion, at least from an aspect), which hopefully might the answer more clarified for you:
For instance, you as the father of your son (as an advice) tell him to
say Salam/hello whenever he visits his friends/relatives, BUT:
It doesn’t mean that it is wajib for him to say that, but it can be
regarded as a recommended practice which indicates the politeness of
him, otherwise it doesn’t mean that he has done a sin by forgetting
(not saying) that.
As a conclusion, such using of words/phrase in that Ayah seems to be related to emphasizing of monotheism politeness (and being aware about the will of Allah in performing everything/everytime -as much as possible-) but it doesn't mean that it is Wajib to say so.
Albeit my answer might be incomplete, but God willing it helps you to some extent. Of course, there seems to be a complete persuasive answer in the following link (that used it somewhat as my reference), but, unfortunately I found it just in Persian (Farsi):