Is the abrasion blaster mandatory? I have 2 free medium hardpoints and currently think about installing 2 seismic charge launchers for more ammo.
Not mandatory... you could collect only the free floating chunks after the explosion but - techniques to generate multiple chunks per deposit aside - an abrasion blaster increases the yield per asteroid by roughly 75%, so I'd highly recommend it even if it means to return to a nearby station to rearm once every few hours.
The 72 rounds a launcher holds are sufficient to detonate 20-25 asteroids with some practice (see above). I can currently fill the entire hold of my mining python (192 tons) with half a launcher's capacity.