Well, let's stay with BoD.
On the 96% version, the 5,000x rate is 1:37,500,000 spins as the excerpt below shows.
Now reduce that to 1:75,000,000 and you've got your RTP reduction without affecting the rest of the gameplay. Again, 99.9999999999% of players won't notice the difference except the one in 37.5 million as he/she won't get the 5,000x win. You could go much lower than that if needed.
Hyperwins are typically 2 - 4% of the RTP
View attachment 115644
There are many ways how you can increase/reduce/shift RTP. Have an educated guess how much the RTP portion is for the scatters pays on DoA? There is a lot you can shift around without players noticing much.
On DoA2, they shifted appr. 2.5% of the RTP from the base game to the free spins to cover for the additional Hyperwins. You can see it in the base game because you hit more often wins with royals than with premiums compared to DoA.
*17.4%