When I started Aikido, I would often hear the term Aikidoka to reference someone who studied Aikido. I just thought this was a normal thing to do. Then at some point I learned that adding "ka" to the end was something that should only be used when someone was very accomplished at something. That is to say you wouldn't add "ka" unless the person what quite adept at the art. I also learned that you should never reference yourself and an "Aikidoka" because that's being boastful, and only others who really respect your ability should refer to you that way.
Here in California, I know that no one minds you calling yourself an Aikidoka. And we call other people who do Aikido, even if they are not yudansha, Aikidoka. Would this seem very strange to your average Japanese person?
As explained to me, "ka" is a reference to a "doer" of something as it pertains to the level of involvement. It would be a polite suffix expressing that you are involved sufficiently in the activity. Much like our designations of employment. It would be appropriate for someone to call you "aikidoka" but not to call yourself one.
Related was a claim that karate and judo applied this term primarily for those who competed. The implication is that [successful] competition was some marker of involvement that was acceptable for the connotation. This part is hearsay and could be BS.
For the last many years, I have not heard the term used consistently enough to apply a grammatical rule. For me, I think aikido is a universal word and I tend to use "aikido person" or aikidoist because I am not a native speaker.