As you rightly imply, there's no absolute "degree of imperativeness" concensually accepted by speakers for any of these modals.
Where it really matters, these words should be treated as terms and defined, just like any other terms in the contract/specifications in question.
Two slightly anecdotal comments that I can add from my own work. Firstly, as a professional programmer, I have dealt with formal specifications from time to time, and these generally stipulate the intended meaning of "shall", "must", "should"; "shall" and "must" are typically treated as synonymous, while "should" is interpreted as a "nice-to-have" or something that "must be implemented if readily possible". From that point of view, the answer to the previous question that you mention is, I think, wrong.
I also translate (among other things) contracts for a living and have consulted with lawyers/paralegals from time to time on this kind of issue. Amongst the legal proefssion, there appear to be two conventions readily used and accepted: (1) use "shall" for essentially any contractual future event, or (2) both "will" and "shall" can imply a contractual future event, with "shall" implying something more like 'will have a duty to', but with there being essentially nothing legally hinging on the choice of one or the other. In French, the language I translate from, the difference is marked by the future tense of the verb 'devoir' vs the future tense of other verbs, so I concede that part of the reason that I make a distinction is simply for "translational convenience". One comment I received from a property lawyer I worked with recently was that they preferred to use "shall" throughout because it "implied more of an obligation", but conceded that it was just a matter of preference (for the aforementioned translation reason, I was suggesting using "will" instead and they agreed that, legally speaking, it made no difference).