How to ensure a specific request is run by all users at the same time in my test script?

I am setting up a test where I want to "isolate" the last part of a prosess. I know there are some heavy requests during my test plan, but those are necessary for running the last part of the process. I am afraid the heavy requests might clutter the result of the last part. I would like to make all the threads (about 500 threads) do all these necessary requests and then wait until all of the threads are ready to execute the last part of my process, when the load on the server is not too big.

I have not tried anything as I am unsure of where to start. I have researched some, but as I am new to testing, I might have missed an obvious answer.

You can use Synchronizing Timer to create a "rendezvous point" so JMeter will wait until all 500 threads are available and will run the next Sampler with 500 users at the very the same moment

If you want JMeter to run each sampler with 500 users - move the timer 1 level up, see JMeter Scoping Rules chapter for more details and A Comprehensive Guide to Using JMeter Timers for more information on JMeter timers concept.