Usually you want to keep Thrill on cooldown as much as you can, remember it also increases your max HP so you have 20 seconds to actually take the damage before it wears off. If you pair Thrill with Convalescense, they technically become a 20% mitigation cooldown.

For Upheaval, same story, just keep that cooldown ticking If you use it within the duration of Thrill (and there is no downtime preventing you from doing so), they'll always be back for eachother when they come off CD.