Normally there is 30 seconds of latency between opening Lightroom Classic and the sync status check where anything would start syncing. I have found it is ample time to pause Sync.

If you are really worried about this, disconnect you computer from the internet while launching LrC and Pausing Sync, then turn it back on.

Depending on how you connect this could be turning off WiFi on yuour computer, unplugging the RJ45 cable, pausing your router, among others.