We just went to Bermuda on Adventure. The Antigen test is needed within 2 days prior. So if you leave on Saturday, can be done Thursday, Friday, or technically early Saturday.
We were able to get it done at a CVS drive-thru (however you are correct it is getting more difficult to get these appointments). On a previous cruise we self-tested using an online proctor and that worked fine. We used Rapid Test and Trace.
Enjoy your cruise.