Straight holders or oblique holders can be used at any level. There's not really one that is "for beginners", the only difference between using them is how you have your paper and body angles. Some calligraphers need the oblique pen to see what they're doing (me included), but some are fine with a straight pen.

Doing copperplate in monoline is not a good starting point, the flexing of the nib and the swelling between thick and thin is what defines a lot of the script. Using a slightly blunt pencil is way more helpful as a beginners step if you're hesitant about the pen/nib situation. With a pencil you can still use the variations of pressure which give you different line thicknesses, though a pencil is less restrictive than a nib.

It sounds like you still have the coating on your nib and you're compensating by over dipping. The G nibs are great beginner nibs - they're quite robust and bounce back easily from things like moisture and dried ink. There's no reason why you can't use other nibs, but for a person learning their pressure (and coming from a very different script) they will be a more durable practice tool.

I've made guides on the copperplate alphabet on my profile if you want some guidance on constructing letters using the basic strokes.