1 Koveid or Hazak BOTH refer to Paroh being stubborn. The terms are interchangeable. This seems to be the method of classic commentators.
2 Koveid means something else entirely - viz. HEAVY.
In Egyptian Mythology a Par'oh's heart was weighed at death. When light as a feather he earned "Heaven" otherwise..
Thus a HEAVY heart does not mean Stubborn but rather Evil
This p'shat might have some advantages
1. It is more literal
2 It matches what we know about Egyptian Culture
3 It places Israel in Egypt at the Exodus despite the "critics"
4 It distinguishes the 2 terms
1 It's NOT traditional
Pick your peference