Actually, due to the corruption system, it is impracticable to exploit planets with good deposit and 5km size.
My suggestion is to add a new mining module that allows exploit planets with a worse efficiency than mines.
Here an example how this module could work, the numbers are fiction and need to be balanced.
The planetary deposit depletion is fixed at 1000 per mining module per hour.
There are 3 tiers of mining modules with 50%/62%75% efficiency
The mining rate (not depletion rate) is multiplied by the planets percent values, ex 30% Beron.
In the case of 30% Beron, and Tier 3 mining module, you would gain 1000 * 0.3 * 0.75 = 225 Beron per hour while the deposit deplets with 1000 per hour.
Planets like this could be usefull while it will never get colonized by bios.