Reporting issues to HR during these unsure times in regards to management?

As a supervisor, myself and my department have been dealing with a very “abusive” manager for about 3 years now. There have been complaints to HR in which I know the manager was talked to about but nothing came of it. The company laid off about 70% of the employees due to the COVID-19 impact and now there are only a few left. I’m one of a small number who this manager trusts and they have asked me to never go to HR with an issue, to always go to them, and that they hope they can trust me. I’m not going to get into the details but I’m extremely conflicted. I’m very grateful to have a job during these times but I feel like this could be my only chance to put the truth out. What if me writing an email to HR gets me fired? What do I do when I’m dealing with literally, a horrible boss? I have held a professional composure but I feel as if now is the best time (for the manager to be let go), yet the worst time for me (aka shooting myself in the foot).

Not sure that many will read this but I’m hoping some respond. Thank you!

Signed, The business professional looking for help.