You saw the official rules…..
What I have done is booked the first 3 nights using cash or points and upgraded to a suite using cert. Then 4th night using a cat 4 cert that is expiring soon on a separate booking knowing we may have to move room on day 4. I asked WoH to note my free night confirmation in the first booking so hotel can see the back to back stays. In 2 occasions that I can think of, hotel extended the curtesy and we didn’t have to change room. YMMV.
I tried the "booking 2 reservations" trick. I first booked several nights using points and successfully applied a Suite Upgrade Award. I then booked another night using a Cat 1-7 award. When the hotel reached out to me in advance of my stay, I mentioned the 2 reservations. The hotel then removed the Suite Upgrade (and returned it to me) on the first reservation. I couldn't argue as what the property did was in accodance with the T&Cs, but ...
YMMV, indeed.