Plugshare shows everything, including sites that don’t exist or are misplaced (it’s a crowd sourced database), and AC/slow charging sites or sites that aren’t built yet (although you can filter those out).
Tesla Nav shows all SC sites plus a subset of third party sites that meet its listing requirements. You’ll see most Evie, NRMA, BP Pulse etc sites there but it is a bit hit-and-miss. Tesla Nav will show how many stalls are free at all SC sites (or a clock if there is waiting) but it doesn’t have that information for the third party sites (you need to separately look at their apps, or Plugshare if the other networks feed their occupancy data to it).
The other thing to note is that third party sites are often only 1 or 2 stalls, greatly increasing the likelihood all are in use when you arrive (Erlang queuing theory and all that).
Generally Tesla SC sites don’t have loiterers due to Tesla’s idle fees. However Tesla SC sites open to other EVs can suffer from EVs that can’t charge very fast (e.g. might have a limit of 120 kW or lower) but note that Tesla does not open its busiest SC sites (mostly on major highways) to other EVs. They remain Tesla only.
Only the more out of the way SCs are open to all, or the ones that have been government subsidised but those usually have way more stalls (e.g. Raymond Terrace and Taree both have 12 stalls).
For your first interstate trip, I suggest you stick to Tesla SC sites, with the others as a backup only. It will make your life simpler.
Enjoy your trip!