There are plenty of 6mm eyepieces out there. One issue you might want to consider is that with small eyepieces you need to get the eye nearer the eyepiece. Also looking directly into the centre of the eyepiece is more essential. They are a lot less tolerant when you move from side to side causing the image to disappear.
Getting say, a 14mm eyepiece will give you 1500/14 = 107x magnification which would fill a gap in your range. With a barlow that would be 214x.
It may not be a issue with you, but with my aging eyes I tend to use a barlow.
re the 10mm eyepiece with the barlow giving 300x magnification, there is no harm in trying, but if it is too wobbly and/or dim, you will know why, and you can go down to a lower magnification. Basically that is the procedure. Increase your magnification until it becomes too dim or unstable to view, and then go back to the previous magnification. That will be the best for that night's conditions.
Consider Philip's advice too about getting a focal reducer to get good, bright, wide angle views of larger objects.
I use an 11" reflector (Celestron CPC 1100) and a 3" refractor, (Sky-Watcher ST80) mounted on an equatorial wedge, housed in a 2.2m Pulsar observatory. I use a ZWO ASI 120MM, ZWO ASI1600MC and Canon 1300D for imaging.