How can I avoid Time Machine starts to back up files right after the Mac is restarted?
I've found two TimeMachine schedulers softwares: TimeMachineEditor and TimeMachineScheduler.  

Both are useful to set the backup interval of Time Machine but none of them enable user to define a time limit (after mac startup) before which Time Machine can't be launched. 

However, TimeMachineScheduler has an interesting option to skip backup during a user defined time slot. That may be a decent workaround if you're able to define the time slot -during which you may be "doing something urgent"- that requires Time Machine to be off.