I sometimes write little bits of lore and fluff for planned projects, and the way I usually go about is thus:
1. [Main System Name] 'Prime' Sector - eg [Corona] Prime Sector
2. Creature/God/Location from Mythology/History - eg. The Nemean Sector, The Troas Sector, Achaea Nova, New Tarentum, Zeus-Soter Nebula
3. 'Groupings' - eg. The Adrasite Cluster, The Forrod Enclave, The Branx Trail
4. 'Fortification' type names - eg. Gaborina Bulwark, Pesch Gateway, Uindara Redoubt (Those are actually names of sectors I created in my latest Stellaris game

)
5. Sometimes I'll give it a name that indicates a 'historical' event or person - eg. Jorn's Folly, Bor's Ascension
That's probably not so helpful regarding
actual Imperial naming conventions - but I don't think
GW really expected people to want to know that level of minutiae.
(Strangely enough, I'd recommend playing Stellaris for naming Sectors - the specialisations of planet economies, situation of starbases and the way hyperlanes between systems intersect gives sectors interesting shapes and aspects - so you can get quite creative with names. Bit
OTT perhaps, but I like me some immersion

)