Brad, no-one can say that you haven't given the oblique holder a fair chance after 8 months, and you're under no obligation to persevere with it if you don't get on.
I'm neutral about this business of whether an oblique holder is suitable for roundhand because I have no copperplate experience whatsoever. As you say, plainly a straight pen or a quill will work, so the answer from IAMPETH sounds rather doctrinaire to me. If memory serves, in addition to the things you mention Mr. Antonio also says that the line of universal beauty has a tendency to change into an asymmetric Spencerian capital stem shade with an oblique. There may well be a grain of truth in that, although frankly, I wish that tendency was more pronounced.
There's a strong case for right handers using an oblique holder for Ornamental Penmanship - in fact the style is partly defined by it - but otherwise I see no reason for putting up with discomfort if you're getting results with a straight one.