on a completely different subject.
We need heavy shock troops, correct? for most other uses we have the stems/saps (main troops), dragons (Heavy Air), drakes (Ground support), firejackets (Light air) and spiders (light round support).
Now, think of a tank. A Main Battle Tank.
Now, use life magic to create a bio-machine that looks and operates like a real life tank that can be manned by stems.
They could either fire shaped explosive charges (our alchemist is already making the explosives) or fire flames or spores from its cannon.
It's heavy armour (heavier if we clad it in steel over the hull) would be good against the hoards of he EoT and Maher.
Even if we accept your premise ("We need tanks"), why would we make bio-machines?
We could make them fully biological, which removes the need for Stems to man it and also fits with our current feats better (not to mention isn't so anachronistic).
We could make them fully mechanical, reducing complexity and maintenance as well as protecting them from death magic.
Making them a weird hybrid gives them the weaknesses of both and few advantages of either.
About the trogs, how about instead of making them like serving us just make them enjoy working hard. Sort of like how dwarves are industrious, these could be similar. It doesn't force them to work for us or to perform specific tasks but it does make them try their best at everything they do which would make them good citizens.
I can dig it
I like the idea of making them
motivated, or high achievers. Rather than restricting their abilities, it encourages them to reach higher and higher, helping them improve their position. I approve.