I leaned (again) heavily into an older British organizational form. It's a regimental system at its core, with a Ground Forces Command, a Close Orbit and Aerospace Command, and a Nautical Forces Command - nature of the planet might shift some things around somewhat. There is the Regular (Line) Army, the Territorial (Reserve) Army, and the Colonial (Militia) Army. The Regular Army units are often named after the sponsoring noble, subsector, or sector, while the Territorial and Colonial Army units are named for the planet or system.
There are occasional "Imperial" RA units that tend to be more elite troops, and some TA units are designated as part of the Imperial Ready Reserve and organized and trained for quick deployment to other systems as needed.
There are also odd groups like the Imperial Cryogenic Reserve (aka the "Methuselahs") that stays on ice for the majority of a 4 term, waking only for deployment (mostly in emergencies) and skill/training updates.
There is also the Orbital Guard (aka Coast Guard) that performs similar functions but is a distinctly different from the Imperial Navy.
Nobles may have Huscarles, which vary in size and may or may not be included in the TOE of the Army (often not, but get added in when war is declared), and the knightly Orders Militant also maintain independent fighting forces.
Marines are a much smaller, much more elite force - topped by the much smaller and equally elite Imperial Guard (the Emperors Honor Guard), which is itself topped by the very, very elite Imperial Cadre (basically the Emperor's Huscarles).
D.