Make the Protoss Photon Cannon convertible into a shield battery instead. Kind of like the Command Center into a PF or into an OB.
After the photon cannon, a proper amount of gas will convert it into the shield battery. This way protoss cannot abuse cannons and shield batteries, it will balance out the investment.
Requirement: Forge
Upgrade Cost: 50 Gas
Pre-Requisite: Photon Cannon is built, selectable and holds this option.
Conversion time: 20 Seconds (a very forgiving build time, enough time after scouting the enemy).
This way, protoss cannot just warp in a shield battery. As a result they cannot just warp in cannons and batteries simultaneously.
Balance requires a trade off of one or the other. Protoss survives any counter attack via recall, and the ability to warp in a multitude of units if they are out of position.
You already have recall.
you already have warp in.
You now must go through the tree of cannon, into the battery from the cannon.
It will only be 50 gas. and 20 seconds conversion time.
Protoss units already have shielding. Therefore gas is required for a building to heal the shielding.
Terran builds bunkers but it needs to be manned, the turrets only shoot at air, and Zerg requires drones to build the spores and spines.
Protoss needs a pre-requisite before the shield battery, and must trade off something because it heals the shielding of units. The overcharge is then justified due to the pre-requisite.
So the choice is now to damage the enemy, by defending with a cannon, or to heal your units while they are being damaged.
So for example, the shield battery would buy time for protoss units to recall home, or warp-in. A much more balanced scenario if a base is attacked/probes are being hit.