The email protocols don't natively support this, so yeah your best option is to embed or link to a webpage that becomes inaccessible after a set amount of time.
MP3 metadata management has always been a pain in the ass. I can suggest Picard and Beets as useful tools, but there will always be some effort required to maintain properly
Then either you've been living under a rock, are very young, or the algorithm of the platforms you use has not been showing you that, because it's been openly used as a marking on their military vehicles since day 1 of the Ukraine invasion, and has become a symbol for both sides of propaganda.
Or when you spent "long enough researching in detail" you somehow missed the Wikipedia page for it (first result in Kagi and third in DDG for "Z symbol" - link doesn't render properly on some clients because it has parentheses)
Let's not call being ready to give your life for a cause being suicidal. Desperate, sure, but not that