Thank you everyone for the suggestions.
It does look like my commander's loadout needs to be carefully optimized. It's almost at level 6, but a Felon or a Stardust turret can take it out easily if it's out of the range of the disintegrator. And I definitely will make more mobile AA. Now it's going to become all about stopping the shieldbots' advance before I get to their factory. ...And maybe waiting for the difficulty to get tweaked if that doesn't work. ;)
Sometimes I wish Zero-K had something like the Avenger in C&C Zero Hour to counter Racketeers. If you haven't played that game, it's a combined AA/antirocket unit that shoots non-pulsed lasers and has no ground attack. Somewhat overpowered IMO, but fun.