When I realized we were gonna do some Missile Command action, I had hoped for some cell fracture/Asteroids kinda fun, but accept that we all have to start somewhere. Here's a link to a free Unity asset that will take your falling rocks up a notch or two, and then check out 'Cell Fracture' in Blender to be able to make your own!
https://assetstore.unity.com/packages/3d/props/breakable-asteroids-167825
If you wanted to expand on this concept further, I'd suggest perhaps having a multiple grab setup for a large cannon like you have (to reduce the wiggle of this 'heavy weapon') and/or options of one big gun vs. two smaller guns (one for each hand?) Maybe the rocks that hit the ground spawn enemies that charge you at ground level, to vary the angle of what you're shooting at? Nice one!
