A ˝ HP pumping from 300’ won’t deliver much water. It could even be a 5 GPM pump at that depth. But it doesn’t matter how deep the pump is set, what matters is the pumping water level in the well. Even though the pump is set at 300’, it is only lifting from the actual water level in the well.
The fact that the pressure still hits 60 and shuts off the pump when a shower is running means that the pump can deliver more than you are using. But if you add extra showerheads, that may no longer be the case.
A CSV can control the flow from the pump to match the amount being used. This would keep the pressure at a constant 50 PSI for as long as the shower is running, instead of dropping to 30 and spiking at 60 several times during a shower. 50 PSI constant will seem like much better pressure in the shower than when the pressure is bouncing between 30 and 60. However, the CSV can only do this if the pump is large enough to handle the amount of water being used. If the pumping level in the well is only like 80’ deep, then a 10 GPM, 1/2HP pump can supply a lot of water. The deeper it is to water, the less the pump can supply. I would test it out by running enough hoses to keep the pressure at about 50 PSI, then measure how many GPM you are getting in a bucket.
If the pump can supply enough water, then the CSV with the 4.5 gallon tank is all you need. Adding a larger tank will not help, and will actually make the pressure problem worse, as the pump just sees the big tank as another demand it has to fill.
You can’t purchase a regular size tank for what the complete Pside-Kick kit will cost, and the CSV will work better than a room full of pressure tanks.