A few potential ideas for rewards I've thought of for this goal.
Unlockable Cargo Crates
A couple of select cargo crates that only become available after certain point thresholds are met. The first few could be ones that aid in further drilling, such as Oupost Defense crates with gear for fighting off enemy swarms or defending existing pumps. Later crates could be things like exotic material crates containing diamond and bluespace crystals, to make up for mining being distracted by defending the drills.
While drilling, there could be the possibility of unearthing valuables and treasures. The most common would be mined materials, but there could also be a chance of unearthing Necropolis Chests, and in extremely rare cases, a boss chest.
Rare Enemy Spawns
Drilling could increase the chance of rare fauna spawning, giving those xeno-collectors a fun time trying to catch 'em all (And then force them to defend the very thing they sought to destroy.)