Learn English – So is everyone else

Phew, what an example...

Let's use a shorter example with less tech speak.

• I'd rather my Mom cook dinner tonight. or
  
• I'd rather my Mom cooked dinner tonight.

Pattern:
Subject 1 - would/'d rather - subject 2 - verb - ....

This is a slightly more complex pattern that the would rather - phrases with the same subject (subject - would/'d rather - verb in infinitive/subjunctive); e.g. I'd rather have a cookie (...than ice cream).

This pattern is used to express a preference, but note that the alternative is not always stated expicitly. Quite often, it has to be inferred from the context.

Please note:

• In AE, both present subjunctive and past subjunctive are gramatically correct.
  Hint for learners: The present subjunctive seems quite similar to the simple present. It can best be recognized by the "missing +s" in 3rd person singular. (-> ...Access take care of...) The past forms are identical. Only exception: The verb "be" has two distict subjunctive forms: be (present) and were (past).

• In BE, for two different subjects:
  The verb in simple past is used to talk about events in the present or the future.
  For past events, you need to use past perfect.
  See more details: Cambridge University

Therefore, your original example could also be:

But I'd rather Access just took care of making sure that my food order IDs are unique.

The simplest way of indicating a requirement, as per the RFC, is the use of the word "must". The alternatives mentioned in the RFC exist to allow for slightly more natural English avoiding the repetition of the word "must"

"Every car MUST have a key, which shall be made of metal and is required to weigh less than 100g"

The RFC was trying to codify existing practice, in which words like "shall" were already in use. The trouble with shall is that it can indicate the simple future (usually with a first person pronoun). Saying "The car shall be delivered on 10 July" could be an obligation or it could be a rather badly phrased prediction (will would be better here). Technical specifications don't need to be great prose, so just use "must"

There are a couple of notes about your question: You say "They are pretty much recommendations (i.e., we do not impose anything)". The purpose of the terms "MUST" or SHALL in the technical language is to impose obligations. The car MUST have a key, or else you don't get paid. For recommendations you use SHOULD "The car should be painted blue" (unless there are good reasons not to.)

Also, you say that you are writing these "for customers". Normally it is the customers that write the specification. Do you mean "for suppliers" or are you actually talking about technical documentation, for which the specification language of RFC 2119 is not appropriate.