WOW - great aspirations you have!
I say "go for it" while you have time, energy etc - I wish everyone had such a passion for self development and ongoing education.
But back to your point.
To answer your question you have to look at what the "core curriculum" of transport nursing is - I think in there you will appreciate what this encompasesses for the job role.
For the most part - where transport nursing is involved, there is an expectation of critical care (not ER) skills. There MUST be a highly proficent knowledge of vasoactive drips, ventilators and multi system pathologies and trauma. There is also going to be some expectation of things like post-open heart transports, or active STEMI transports or even balloon pumps - to name a few.
I would encourage you to keep on the path you're on. I wouldn't worry about the EMT/Paramedic thing that much at this point, it's not a necessity.
If possible I would spend time in a critical care class to get exposure to those drips, ventilation management and the multiple system pathologies. I would even go as far as to say that the CCRN and CTRN certifications would take you further than the CEN one would in the world of transport nursing.
When/if you worked on your EMT-P, then you could add on a CCEMT-P oor MIC-P course - this is probably a more effective course and certification than any critical care nursing transport course out there.
I hope this helps.
Feel free to email me privately if you have any other questions, need any other resources or information.