In addition to what Jim said.
1. You still need to learn more about how Playing Now works in MC by the sound of it. Everything you choose to play ends up in Playing Now, and then the transitions Jim mentioned apply. So there is no transition "from one playlist to another", as only the contents of Playing Now get played.
So you add Playlists or Tracks to Playing Now by the various methods available, and then they sit in Playing Now in sequence until MC gets to them, and it transitions as per your settings. You can resequence Playing Now quickly. If, for example, someone requests a track that you have later in Playing Now, you can just drag it up to be the next track after the currently playing one. Or you could insert a completely new track, or add one to the end, if say a wedding party requested a particular track for the end of the celebrations. Use either a Split View, or tabs in MC to be able to manage Playing Now while still looking up Albums, Tracks, etc. during a gig.
2. Just Clear Playing Now at the end of the night. If you want to remove all items from a specific Playlist, just right-click it and select 'Clear "[Name of Playlist]"'.
Smartlists can be set up to add a certain number of Tracks according to rules you set up, then you just add the Smartlist to Playing Now to add that number of Tracks. Each time you refresh the Smartlist you will get a new set of Tracks, although there may be overlap, because of the rules.
But I assume as a DJ you individually select and sequence Tracks to play, and don't want to use automatic rules. So if you have multiple Playlists that you may want to select from, or even just one big Playlist, then just select the next lot of Tracks to add to Playing Now and you won't have a huge list to scroll through. Or just add your huge Playlist, and clear Playing Now at the end of the night. That sure isn't an issue.
There is even a setting, "Options > General >Advanced > Clear Playing Now on exit" that would do it for you. But I don't recommend turning that on for a DJ, as if you have to restart MC at any time during the night it would clear your queued up Tracks from Playing Now, rather than just allow you to continue from where you were.
Do look at the setting "Options > General > Behaviour > Remove files from Playing Now after they are played", which will do what it says and just leave unplayed Tracks in Playing Now, so you won't "lose your spot" if you scroll around a lot, click on other tracks, or have to restart MC during the night.
Basically, practice a whole lot more in using MC for DJing, and learn its functions.