I have not even tried the rockets. Just saw them as a-historical and inefficient.

Howitzers? I have not found them useful for anything except "guiding" the enemy towards my center (by bombarding the enemy's flanks) and quickly destroying buildings with enemy troops. Most of the killing is done by direct artillery though.

For a meat-grinder experience, I'd suggest to install the smoke and blood mod and put an integrated artillery line (3 line infantry + 2 puckle guns) across the diagonal of a star fort in some choke point that the enemy attacks often + some trash infantry to guard the backs of the described killer line. AI's armies will melt like snow in the spring... Be warned that after a couple runs it becomes utterly boring.