To let in "Let there be light." is used to make a third party imperative sentence. It can be rephrased to
I command (order) that there should be light.
It doesn't convey a sense of desiring something, rather ordering something to happen. In other words, it is not desired that there should be light, but there must be light because God says so.
If you contrast "Ask him to carry the bag." with "Let him carry the bag", the former indicates a request, but the latter a command that he should carry the bag.
May is one of the modal verbs in English and it could be used in a optative sentence with which you can express a wish. Your example is in the optative mood which is:
a grammatical mood that indicates a wish or hope. It is similar to
the cohortative mood, and is closely related to the subjunctive mood... One uses the modal verb may, e.g. May you have a long life! Another uses the phrase if only with a verb in the past or past subjunctive, e.g. If only I were rich! Another uses the present subjunctive, e.g. God save the Queen!