Honestly I'm partial to the Jane school of getting good potential inheritors- track down a talented orphan and adopt them.
Admittedly this is a lot harder when looking for martial talent than clmbat talent, since you can't really be accidentally pushed into a situation where you need to command an army as a child, but we have an advantage most worlds don't, aka fucktons of psykers and Ridcully, the best mortal diviner in the galaxy.
Upon askig Durin, an average roll done by Ridcully to sniff out the 'best' adoptable martial candidate would leave us with a list in the thousands. Not great, but that's a low enough number we can either do another divination to narrow down the numbers further, or just take those thousands and work through their rough levels of skill via Gladatoria-based command training. Although it takes up so much energy we can't do this in bulk, we could probably work through the list in a decade or two, or if we're willing to get potentially suboptimal picks, we can randomly select 100 from the list to start with.
Once that's done with, we pick the best candidate, adopt them as a Rotbart, and focus on tutoring them until they're worthy of the name. Optimally speaking we then take that list and offer every kid who's willing a position in a military school because they'd still be talented for the job, but that's optional.
Avernus is a world dedicated to making weapons. As people on that world, we shouldn't forget the potential of any of them, because if they've survived till now, they have something to them. And hey, being relatives by adoption is more than commonplace on Avernus, it's practically assumed you'll have a blood-bond or two, even for Rotbart, Jane's essentially his sister.
The main drawback is pretty obvious, of course. That being that Ridcully's time is incredibly valuable, so we'd always be sacrificing a little to maximise our potential successor's abilities.