One potentially easy reduction fix for starbase spam would be to increase both their exclusion area and area of effect (for economic) by a large amount.  When you play on the larger map sizes, the center of your empire is stuffed to the brim with economic starbases on every available tile, averaging about 3x starbases per planet if you have close together colonies.  For more spread out empires, you can pack as many as 5 starbases per planet which makes the problem much worse.

Fix #1 - Force the player to pick the starbase type (military / economic / mining / culture) when constructing the starbase.  This then allows starbases to have different exclusion radii and opens up the ability where economic starbases do not care about the location of mining or military starbases for their outer zone of exclusion.  There might still be an inner zone of exclusion on all types which prevents placement right next to anything at all.

Fix #2 - Increase the area of effect and exclusion area of economic starbases by a factor of 2-3x.  Now instead of needing 20-30 economic starbases to cover the heart of your empire, if the radius is large enough, you might only need 5-8.

Fix #3 - Apply a stacking penalty (diminishing returns) if you have multiple starbases giving bonuses to a single entity (i.e. a colony).  So even if you could crowd six economic starbases around a planet, you would only get about a 3x bonus from it instead of 6x.  In addition, this means the area of effect for an economic starbase could be 1-2 tiles larger then the exclusion area.  That would make it slightly easier for the player to place an optimal pattern of economic starbases, without having to micro-manage down to the exact grid location (also helps the AI).