Mount the router on a plate rebated into the table and you will only lose the thickness of the plate. By the way, have you removed the plastic sub-base which is normal when mounting under a table. The 2" plunge of your router should be fine for 99% of your routing, a collet extension being un-necessary except for occasional very deep routing.
There are heaps of threads on the forum showing the correct way to mount a router in a table.
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