Fortunately, it seems like the UX experts themselves consider that there is enough compatibility between these two fields so that you can transition from Marketing to UX Desing.
In this regard, you should definitely take a look on this article. It's spot on for what you are asking:
How to Change Your Career from Marketing to UX Design
"Finally, in terms of the job role, which UX position will be more achievable and realistic for my background? Is it a UX researcher or a UX designer?"
As you can see in the article above, "User experience, or UX, is a user’s experience of using a product. That sounds simple enough, doesn’t it? Well, UX design is the process by which we understand what users want and need, and incorporate that into product design to deliver greater user experiences—and that’s the complex part."
Therefore, UX doesn't seem to be a role, but rather, the process itself. UX Design is the role you are about to fill in.
I don't deny the importance of a portfolio, but your self-confidence is even more important than that, especially in a fluctuating field like UX.
In order to increase your self-confidence in this field, the first step I'd follow would be to consolidate my knowledge with some certified online courses, since I don't have the time and energy resources to graduate one more college.It's good that you have followed Dr. Travis's usability course on UDemy. Add something new to the list, to make it more convincing when trying to break the ice by getting that first job. I would go with The Interaction Design Foundation, since they are recommended by Don Norman himself, the man who coined the term "User Experience".
Then, I'd start reading blogs and online journals, just to keep pace with the UX world. I recommend the following sources:
Elite By Design
Centre for Teaching Excellence
Change the Thought
Try to freelance and land a job having the purpose of creating a portfolio in mind. Since you are now a multivalently certified UX professional, people are more likely to hire you. They might pay you less for the lack of experience, but it's a start.
Then, you will evolve exponentially, while increasing the size and the quality of your portfolio too.
If you have any more questions, do not hesitate to contact me.
Good luck transitioning from Marketing to UX! It's a good prospect.