I was wondering if I should wait to go for op8 before or after I beat UVHM?
You need only to start uvhm to safely rank up op. However you will want good gear. This becomes especially important at op4. Playing enough at uvhm will allow to unlock farming areas you might want access to.
In addition to @Chitsa I think it’s worth mentioning that none (0) of the op levels become unlockable before level 72 is your level. Meaning, if you are in uvh mode but at, say, level 55, all you´d gain would be xp, i e, not op. Meaning, however much you rumble at the Digipeak, game will not count it as op unlock less you are at least 72. Hope light shed. / Zerkus.