As an American English speaker, I would definitely not use shall here. The context where I mainly see shall used are in technical standard documents, where it conveys an absolute requirement, not merely a suggestion.
As for should, in other varieties of English, should might convey a conditional, but in American English it will be understood to indicate a preference, making the suggestion firmer rather than softer.
The example as you presented it seems fine to me.
To soften the language even further, you could use could or might:
...it is my suggestion that every product could have more detailed specifications.
Could conveys a possibility or an option, so it makes the suggestion even weaker. However this softens the suggestion so much that it might appear you lack confidence in it yourself.
As was pointed out in the comments, this is not using the past tense, but the subjunctive, which is used to speak about possibilities rather than known facts. In English the past and subjunctive forms of can are the same: could. similarly, the past and subjunctive forms of shall are both should.