Having also spent some time on the topic, let me share my thoughts.

As you said, don't include information beyond identity for the simple reason that nothing is guaranteed to never change, every information you put in there may become outdated. One may think you can surly put the birth country of the customer into its customer number, but what if the customer made a mistake when signing up?

It may be useful to have some information in identifiers, for example a prefix C or O to indicate a customer or order number, but that can be implicit most of the time and just be added in the user interface or when printing labels.

To guarantee global uniqueness one has essentially two choices, enough randomness or unique identifiers for space and time. The first option requires no coordination, not synchronizing clocks and not making sure every process has a unique identifier, the second option allows you to have locality in identifiers generated at similar times or places.

Locality can be useful when identifiers are used as keys in databases or other data structures because it may avoid having every insert happen at a random place. On the other hand this may easily leak information, whether the number of your customers or orders per month or the number of servers you are using, especially if you are using counters, see further down.

One can use identifiers with locality internally but encrypt them before exposing them to the outside, but this comes with an entire new set of problems. One may have to include initialization vectors in the identifiers possibly making them longer. One may have to think about key rotations, at least for the case keys get compromised, possibly making identifiers even longer having to include key identifiers. One will have to do key management anyway and encryption of course costs clock cycles.

If one wants to include a time component, one can choose between actual time and counters. If one uses actual time, you have to ensure your clock never goes backwards and you may have to account for limited resolution, possibly falling back to counters or randomness when identifiers are produced faster than the clock ticks. If one use counters, you have to ensure it never runs backwards, for example persisting it across application restarts including crashes.

If several processes share counters, synchronization overhead between cores may become an issue in high performance scenarios. Modern processors offer high frequency hardware timestamp counters that can be synchronized across cores. This may be especially interesting if the timestamps are also used for purposes like operation or transaction ordering.

Identifiers should be readable if only for debugging by developers. Similar characters like O, 0, 1, I, l, U and V should be avoided. One can consider a checksum or even error correcting codes for user facing identifiers.

Capitalization is a tradeoff between length and ease of use but I like your idea of dealing with it behind the scenes. One probably loses on average less than one bit per character when ignoring capitalization so that for sufficiently long identifiers collisions are still very improbable, at least if one does not use identifiers with a lot of locality. But I fear users will just tend to include the capitalization because they are not aware it can probably be ignored. It surly adds complexity to the application, if only to the user interface, because you have always to be able to handle collisions.