Yeah. I did it exactly once, on vacation, while I didn’t have internet connection/didn’t want to waste the rest of my data, and wanted to import photos to my MacBook to edit them. It was fast enough on the Lightning connector.
This is a complete non-issue especially since faster hardware would probably be more expensive. Apple has enough actual issues that are more important such as repairs, RAM pricing, sideloading, …
Signal is much more polished and less fragile than Matrix, but is pretty barebones especially in terms of features for large communities. Matrix additionally has a browser client and many non-Electron desktop clients. It’s really close and depends on the use case imo. For personal messages I think I prefer Signal, for communities Matrix.
I haven’t used Threema.
INFRA! Just look at the screenshots on the Steam page.
Can you export it as an email archive file and copy it to a USB stick or upload somewhere accessible from your personal computer?