Those are good results! You have good drainage, only difficult holes to dig due to compaction, but tree roots can easily penetrate clay, still I would excavate a large hole and use some of that clay to enrich your garden while mixing in at least a 10% organic material which could be any one of these: compost, composted wood chips (at least 3 years old from middle of a large wood chip pile), spagnum peat moss, or bagged top soil with vermiculite). I would not be concerned at all about drainage with those results. Good luck
Dennis
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