Wow, congratulations ramimpdocs1!
It seems that the 5 year period for PBS dependents can be calculated from the date of first visa issue date, rather than from entry in the UK! I have a similar case and have been trying to find examples where this is true.
In our case, the only difference is that the first entry of my spouse was about 120 days after the initial visa approval date (based on new guidance, they check for 180 days, not 90). I am thinking it should be fine if we try 5 years from first visa date (instead of waiting another 4 months!)