If it is a random event triggered that is all in the config on how often you want it to spawn as it only does it once the timer hits.
Now, if you are allowing players to call them, You should be setting a cooldown for each player at the bottom of the config if you are using Server Rewards (which is how I allow players to call them) you really want to add a cooldown to that. Example I have it as they can only purchase the command every 12 hours. I also do not make it cheap for them to call them since there is already an automated event to spawn in its random times. I wouldn't recommend this as a cheap thing to do as players can abuse the plugin. Think of it like the Personal Heli plugin that allows players to call a heli for them to take on. You wouldn't want players calling the heli every hour to exploit it. Same thing here. Make it costly to call it and an extended cooldown to prevent calling it so much. This allows for a much better experience and allows the automated event to run as intended.
Doing the above should fix most of the issues.
As far as the despawn, you can always increase the despawn times as well.
Slowing the plane down doesnt make any sense to do that. You can also lower the speed and spread options if you want them to be tighter togeter.