You'd do well to ask these on the
https://lists.gnu.org/mailman/listinfo/groff list for a wider set of opinions. It depends on the style and complexity of the document. -ms is simple enough that people like W. Richard Stevens would tweak it for their books. I understand the relative newcomer, -mom, is comprehensive, modern, and well supported by its author on the above list. I don't recognise the Pascal, etc., analogies. :-)
I do write my own macros. They can be just short-hands for a combination of others in the same way my ~/bin/l is exec ls -l "$@", or sometimes for a simple document I start with just troff and have some macros on top of that. Yes, any distinction over semantics is purely convention.
You may wish to read Kernighan's _Nroff/Troff User's Manual_, http://troff.org/54.pdf, otherwise known as CSTR #54. It's original troff, not groff, but as a succinct reference with elegant prose we often refer back to it. At the end is a tutorial introducing simple macros.
Integrating troff and friends in pipelines and scripts is easy. They take line-based text as input and produce it as output, only switching to binary for some output formats at the last hop. You can also run system(3) from within troff documents, e.g. to include the output of a command, but often that's not the easiest fit.
I recommend again the groff@gnu.org list; they're friendly, patient with newcomers, and interested in showing how they tackle the task at hand.