Metadata Card Configurations - Groups

I am surprised to hear that you connect the two. In my understanding Property Groups is a way of grouping properties and sometimes making those groups collapsible for the purpose of making the metadata card more user friendly. Grouping is not required or necessary and it is certainly not related to the number of rules. I rarely use groups and I believe the highest number I have ever used is 3 in a vault that has many rules. The primary reason has always been to be able to either group properties in a logical order and/or to be able to collapse groups of properties that are not used in the majority of use cases but still need to be there for certain rare purposes.

I use the groups in occurrences when I have several properties on a metadata card as a way to separate out the data and keep it more readable. I have also used them for the collapsible or read only purposes, but for the most part it really is just for separation of data.

So I wasn't sure how to manage these best.

OK, now I see what you mean. We agree on the purpose and general use.
In fact I have just created a setup where I used the same 3 groups for 2 different object types (hardware and related documents). Some of the properties were common for both object types and some were specific to one of the object types. I just put all of the relevant properties from both object types into the same group. It works fine, because the metadata card will only show properties that are present on the object at hand. So even though there is eg 10 properties in Group 1, it will only show property 1, 2, 3, 4, 6 and 8 when looking at hardware, and it will show property 1, 2, 3, 4, 5, 7, 9 and 10 when looking at documents.
So you can indeed reuse groups, and it makes sense to do so if they have at least some properties in common.