Perhaps if a beetle targets a building, it should have a different death weapon--one that does more damage, but has much much much less area of effect. Against units it would have a broad AOE and also have an emp effect, but against buildings it would deal savage damage. And because the AOE would be smaller, I think that makes it less likely the damage would be absorbed by shields.
We could also lower the damage and increase the AOE for the anti-unit weapon. We would want the AOE to be big enough that if a transport dies carrying beetles, it would damage units on the ground. A T2 transport loaded with 10 fire beetles should be a viable weapon against T3 bots, you would just ctrl-k the transport when the beetles are overhead.
Suppose we made this change: decrease their damage to 850, increase their damage radius to 8 (from 6), and add a separate death weapon with 2 damage radius (same as TML) and 2500 damage (enough to one-shot UEF t2 economic structures). For 1830 mass you could fly a transport with 10 fire beetles over some percies and kill them. Or you could drop 10 beetles into an enemy base and kill some T3 pgens.
Or perhaps set their anti-building special attack damage lower like 1700, so it takes 2 beetles to kill any faction's eco structures, or 1800 so they can one-shot cybran eco structures but everyone else takes 2 beetles (since everyone knows we need to nerf cybran).
Also, since they would be more dangerous (with bigger AOE and with a special anti-building weapon) their HP should be lowered slightly, maybe to 270 (same as a mantis, that would also make it easy for players to understand how much firepower it takes to quickly kill 1)
They would be effective against buildings OR against massed units. They wouldn't be great at sniping ACUs (because they would use the anti-unit death weapon) but two transports full of beetles could be ctrl-Ked over an ACU to deal 17k damage. Flak should deal effectively with beetles on transports (it would kill beetles about as quickly as it kills out-of-fuel interceptors, because they would be moving at similar speeds)
It would also let people use a transport carrying 2 beetles as a way to shoot down enemy transports
It would be viable to put beetles on stingers
If they don't already, beetles should have high target priority from other units
This would dramatically increase the number of ways that you could use beetles. I have no idea if this would make them overpowered. Obviously the balance could be fine-tuned from there.