Workaround: The top of each of my notes includes a line dedicated to tags, such as tags: #tag1, #tag2, .... (I actually thought of abandoning this custom since Front Matter was introduced, but haven't yet.) This can serve as a workaround for auto-complete (you can type your tags), copy / pasting (you can copy and paste like any other text), and export (it's always present in the MD note). However, this doesn't allow batch-tagging multiple notes.
You can then convert these inline tags to Joplin tags on-the-fly (if you don't mind the auto-complete) or using an inline tag converter, so that they are properly indexed by Joplin. Alternatively, you may search these tags using Joplin's text search instead of tag search, or use inline tag search.