Efficiently regulating primary cells for a long-life IoT device
OK, you're worrying about overvoltage.

I've never designed something like you're doing, but I wanted to focus your attention on something you could have forgotten: temperature. Temperature is something that has a tendency to behave like its other name is Murphy.

I know nothing about Lithium Thionyl Chloride, so I googled it, clicked on the first link, then clicked on a random datasheet for a random cell...

And whaddyaknow, the open circuit voltage rises with temperature at an alarming rate.

Will your stuff be exposed to direct sunlight?... 

I would advise using a high-current CMOS micropower LDO.

High current, because you want the pass device to be enormous, so its dropout voltage is tiny under load.

CMOS and micropower, because idle current isn't your friend.