Tristen Is Not A Lost Cause
Rhea knew a lecherous person like Tristen would concoct beauty pills first, probably to beguile the women with it.
Damien could only concoct Rank 4 Pills at most since he was in Tristen's body. Even if he somehow manifests Fallen Flames, it would be nowhere as powerful compared to when he manifests them in his original body since the amount of his soul essence in Tristen's soul plane was too little.
Also, because of that, his pills wouldn't be Supreme Tier and would reach only 90% potency at most. But that potency level was already considered excellent in this empire, so he wasn't bothered by it.
And even if he hypothetically concocted Supreme Tier Pills, it would only put too much attention on him and might raise suspicions.
Still, if he wanted to, he could just have himself concoct some Rank 5 Pills during the night in his original body and have them sent over to the palace so that he could accept them as Tristen.