add an adjustment to the "XP" modifier, the ability to turn off the experience supply of the selected tower, which, let's say, was installed earlier and reached the maximum level, thereby it will not take half the experience from the modifier
add an improvement to the research branch, reducing the experience submission time from 10 seconds to 7
Use a Balance modifier or plan better. It is already a very strong modifier as is.
The second suggestion doesn't even refer to anything as far as I can tell, as XP is immediately given when a resources is mined
