When talking about lore I think its important to remember that the "low tech" ships we see aren't actually lower tech than the high tech ones, despite the name, because they've been retrofitted with advanced systems many times. Its also heavily implied that the domain itself has been in a technological plateau for a very long time. The tech levels are much more differences in doctrine and design philosophy than actual changes in technology.
Oh yes of course, Onslaughts were designed before shields were a thing, but that's what begs the question, why would those metal buckets even be in use at this point? Its like a museum piece going around the battlefield. I would get it from specific battlegroups like the XIV, however to expect such a cohesive and relatively regressive doctrine from an entire intergalactic empire sound a little weirder.
They sound like space knights in shining armor, romanticists of sorts. Sword is better than musket or something like that.
I like to think of the XIV as hardcore "old stuff was better" sort of people, and that could also be one of the reasons why they got sent to some backwater sector in the middle of nowhere. Domain was dishonest about AI use, Domain was secretive and obscurantist, that's why even in a place like the academy they couldn't keep such huge parts of the knowledge like terraforming, as such even their doctrine could be a publicity stunt about how "flesh is better than metal" while they built things like tesseracts, sent AI all over the universe to gather data and prepare the ground for non-stop expansion. Chaos in a society doesn't eliminate high tech data backups like how an old library could go up in flames, that requires willing desire to hide and destroy knowledge, someone is responsible for that.
