There's no fellowship for "interventional psychiatry", since nobody wants to do ECT as it reimburses poorly. The efficacy of ketamine is TBD, and the availability now of Spravato will likely render this modality obsolete in a few years. TMS is used frequently but the procedure is trivial enough that you can learn it in one CME session--at most one week-long trip to Bermuda....
There may be more interventions on the horizon--stellate ganglion block, non-invasive vagus nerve stimulator, ketamine/hallucinogen facilitated psychotherapy, etc. whether they'll be lucrative enough to be billable through insurance or frequently enough in cash private practice to make profit is yet unclear.
You can become a straight pain specialist from psychiatry and do things like pressure point and fluo-guided steroid injections. However, pain groups tend to also do other more lucrative, invasive procedures like spinal stimulator placements--which you can learn, but if you are not used to do catheter-based procedures it's a very steep learning curve--these are risky procedures. The employability of psychiatrists in a pain group is low-moderate, and I suspect given how non-procedure based psychiatrists can now earn a very nice salary doing straight or subspecialty psychopharm (i.e. child/addiction), this is route quite uncommonly pursued. Psychiatry is becoming more known as a well-paid lifestyle field, but it's a rare case where it has little to do with procedures.