welcome Duncan
As daniel mentions 300 x is often not usable, in my 12 " ,(similar focal length)
to your 14 ",I rarely use a 5mm eyepiece not you won't ever, just most of the time seeing dosn't support its use that often.
My most used magnifcation is around 115 x 13/14 mm
theres some decent quality widefields around this F//L that wont break the bank or conversely spending big this is where I'd look .
I find the highest power I regularly get use with around our neck of woods is
170x 8.8 mm, below yeah a bit , and i have a couple of not to pricey high power eyepieces to fiill that gap but honestly don't use em to much .
best wishes ,, the plossls arn't to shabby for now either,, good luck .