Inky, what I think I saw in your flexible nib samples is you struggling a bit with pressure control. I'd say keep writing with those flexible nibs, trying not to shade at all. I suspect this will make you adjust your technique towards the (even) more classic method, which will eventually make it easier for you to execute your Spencerian.
Perhaps even better, see if you can write with an extra fine, flexible dip nib, like a Blanzy 2552 or a Perry 27, or something similar. These nibs are totally merciless and will show you exactly where your room for improvement is. In any case this is the reason I currently do my Business Penmanship practice with these myself.
Also, you might want to switch to wider ruled paper, think .5 inch line spacing. This will give you a bit more room to dimension your letters vertically.
Finally, I would personally not switch to an oblique at this moment, unless you really want to work on your shading too, something I would park for a while.
Hope this helps!