[Wavytone]

Pete,

+1 regarding Bintel, excellent advice and service, always.

I'm guessing the focal ratio of your dob is around f/4.5 or perhaps 5. 30mm is about the longest sensible eyepiece focal length for that. The set you have are fine and on most nights 9mm will give the highest useful magnification the seeing allows.

On a very rare night - maybe once or twice a year - you'll get seeing so steady you could use something around 5mm on the moon or planets but it's questionable as to whether it's worth having one if you hardly ever use it. Tracking will also be difficult.

A good choice of eyepiece also depends on your scope - those that work well in a fast Newtonian are probably not ideal in a SCT, refractor or Mak. The converse is also true. There is no perfect eyepiece for all scopes.

For a budget set, plossls should work very well, keep them as it's always good to have a set that aren't too "precious".

Go to some star parties and try other peoples eyepieces in your scope before deciding to buy expensive ones; in particular learn what you like/dislike most - ultrawide field of view vs sharpness, image quality off-axis, eye relief, or detail and contrast on the planets. Each type of eyepiece has pros and cons. Some here have sets that cost upwards of $500 per eyepiece, so yes it can become an expensive addiction !