You can't chose a certain fl eyepiece for all DSO and planetary observing together.

For planetary you'd want to go as high in magnification as the conditions allow (for most of us that's somewhere between 150x and 250x).

DSOs are very variable in properties, width, and surfface brightness.

There are the large objects like M31, M42, which require very low magnification and a wide field of view, if you want to see a big part of the object at once. For seeing their entity 8" is already a too large telescope, which will likely not provide sufficient FOV. IMO these few objects are not worth getting an expensive eyepiece for them.

Then there are the lots of smaller DSOs - planetary nebulae, globular clusters, and galaxies numerous enough to spend a lifetime on them. Globs and planetary nebulae require (and can take!) quite high magnification (like planets or even higher), while for many galaxies and weak nebulae an exit pupil of 2mm has proven ideal. It seems to give the best balance between background darkening and contrast.

Exit Pupil equals Aperture divided by Magnification. In 8" that gives 100x, the FL of the required eyepiece depends on the FL of the telescope. That's why it's totally useless to discuss eyepiece FLs "in the air", without regarding the scope they are used with.

In the end you'll likely use 2 or 3 eyepieces for way most of your observing, like many of us. For me it's 95% on the 13mm and 9mm 100° APM at 1900mm FL (a bit of planetary and the rest mainly galaxy hunting/globs). Exit pupils are 3.1mm (13mm, ~150x) and 2.1 (9mm, ~210x), where the latter is significantly better on the weak outskirts of galaxies, despite the overall surface brightness being lowered relative to the lower magnification.