Congrats on completing your six BCG treatments!! The cystoscopy that you have scheduled will indicate how effective these have been and we will have every expectation that you will receive good news!!
The very best surveillance method is cystoscopy, and your urologist will no doubt have you scheduled for every three months for a while. Other tests that are used only lead back to cystoscopy (and maybe TURB) if they indicate a problem. An MRI-urogram (and the more commonly prescribed CT-urogram) especially examine the entire urinary tract including the ureters and the kidneys to check for any spread into these. My urologist's protocol calls for a CT-urogram every five years "just in case." These are not usually routinely done to check for tumors in the bladder.
As you have probably read on this Forum and elsewhere urine cytology tests can give hints as to what might be occurring, but are not definitive and are somewhat notorious for giving false positive results. Your urologist will prescribe these and the results may only show the need for another check to see if they were accurate.
It is difficult not to be anxious about this, but BCG is the treatment of choice for your diagnosis and is often highly effective for high grade, especially CIS.
Wishing you continued success!!
Sara Anne