Your wiring is ok, but suboptimal. This newer Heltec board has a BUSY bin. You should use it.
This will make diagnostic output from GxEPD slightly more meaningful (BUSY times or timeouts).
The RST connection is still missing.
Your e-paper board may have a different panel than the original one on the web-page.
The GDEP015OC1 is no longer available afaik, and replaced by the GDEH0154D67.
GDEH0154D67 is not yet supported by GxEPD, it will be supported soon. But here is an open PR for a fork.
GDEH0154D67 is supported by GxEPD2. I recommend GxEPD2 for new projects.
Please post the inking on the flex connector. This may allow to identify your panel.