This macro will toggle the entire frame on and off:
/run local t=ObjectiveTrackerFrame if t:IsShown()then t:Hide()else t:Show()end
This macro will collapse/expand it:
/run local o=ObjectiveTrackerFrame if o.isCollapsed then o:SetCollapsed(false) else o:SetCollapsed(true) end
Alternatively, SneekeeMods has an option to auto-collapse at login and to hide it whenever you enter combat. If you want it to stay hidden all the time, I could probably add a setting for that.
https://www.curseforge.com/wow/addons/sneekee-mods