They should give sentry the same abilities (overshield, wall hacks, etc) as the invader when the invader is present, so then there is a true counter to invasions.

Eh

Well first they would need to make the other roles more impactful. They should all be super strong when doing their own jobs. Maybe the collector get double the motes, and during the primeval phase does 3x damage to envoys. The slayer slows and suppresses high priority targets when using a primary, and can gets a recharging overshield when the primeval is summoned. Stuff like that. Second the sentry's buffs are only present when an invader is around. When they're not the don't have them. Third there should be an anti invader role. The player should have counters to the invaders strengths.