I've never found the right tea leaves. Most of them were to bitter and some had just a totally different profile than the original clubmate. How close are the ones you are using?
The taste goes in the direction of ClubMate, but has a stronger tea taste than the original ClubMate. I think reasons for that are the reduced amount of sugar and the fact that ClubMate uses natural flavor in their tea extract.
"All things being equal" = that time constant is itself a function of temperature, so lower temperatures slow down (spread out) the process, allowing a broader "balance point" at which to stop the process.
IOW, if you're brewing a hot cup of tea, leaving the bag in an extra few minutes can really increase the bitterness without making it much "darker", but even a few extra hours in the fridge-brewed tea is no big deal.