I've thought about the same thing as well, and whatever you do, do NOT use any services that may monitor/have AI that detects danger/suicidality/risk of harm because it's too risky and whether AI or some other 'concerned' samaritan may want to virtue signal or flex their power in preventing you from exiting. I would stay away from YouTube and/or Facebook as I know those platforms have anti-suicide AI and what not that searches for red flags, keywords, and what not. I can't speak for other platforms, but as a word of caution, always use discretion and I would try to schedule the video to come up (similar to delayed emails) instead of just uploading it on the spot.
I wish there is a better way to go about sending final messages and letters, but more oftenly than not, there is always some amount of risk involved, like things not going to plan or the wrong person finding out and sounding the whistle.