How about Shield Walls?
People talk about gates like this is swords and horses.
Shield Wall can be similar to current shield except not related to accumulator and not round. Put up two Shield towers, connect them to each other, run off Base power supply and let be target-able (destructible).
Set standings on them so friendlies can pass through enemies cannot
cannot shoot through.
Balance so you cannot layer them (so you can always target the Shield Towers)
add whatever balancing necessary
and ZOOM please test your idea about manually "painting" zones for bases and terraforming. You can more easily deal with balancing zone by zone if for example you discover your painted zone is too close to TP, you can repaint it. Or is someone decided to terraform offensive stuff you can reset terrafoming in that ONE zone without resetting the server. Any issue you find on one island, one zone can be dealt with individually.