[astro744]

How often do you use observe at 300x with the 10mm/2x Barlow and what targets do you prefer? The 230x of the 6.5mm would be used more often I would think but 300x is certainly attainable for say planets with a 12” mirror giving 1mm exit pupil so a relatively bright image. Seeing and quality of optics (surface accuracy and collimation) will be the determining factors.

What about something in the 12-13mm range which you could then Barlow to get 6-6.5mm? Or replace the 10mm Plossl with a 10mm Pentax which you could Barlow to get the 5mm you desire.

If your only issue with Tele Vue is cost then consider used. There is a 13mm T6 Nagler in the classifieds at present or at least was last time I checked. This one would be fantastic in your telescope for large globular clusters and nebulae as well as planets both Barlowed and non-Barlowed. (Quality of Barlow will impact on quality of view).

There is also a used 4.7mm/82 Meade for sale in the classifieds. Not sure if you’re considering this one. I don’t like recommending eyepieces I haven’t used and I haven’t used either Pentax 10 XW or the Meade 4.7/82. I have used the Nagler 13/T6 and it would perform very well at f5 with the only aberration seen being primary mirror coma which you could remove with a Paracorr. (The coma would be tony and for most non-objectionable at f5). The would be no astigmatism from the Nagler. I’m sure others can vouch for the other eyepieces if the Tele Vue is still too pricey used.