The phone always works extra hard after an update. It is normal to experience reduced performance and shorter battery life for the first while after an update, until everything has settled down. Especially an update from iOS 18 to iOS 26, "since pretty much everything" is replaced.
Your iPhone will definitely perform fine again. Especially when you say that the battery capacity is 97% - then the battery is in top shape.
Reducing the transparency is probably more visual than it actually has any effect on battery life, but it probably means a little. What you can try is to restart your iPhone a little more often at first.
But the best way to fix it, but which also takes some time, is a factory restore of the entire phone, preferably done from a computer, and set it up again without using a backup, as everything that was taking up space from before will just be taken over. A clean install simply.
I always do a clean install when we go from a major iOS version to a new one, for example from iOS 18 to iOS 26. I almost never experience any problems with my iPhone, either in the software or the hardware.