This is why the Fediverse and Open Source are so cool. Everyone jumps in to try and make your life easier and more enjoyable.
Made an iOS Shortcut to let you follow someone from a different Mastodon instance without the copy/paste hoop-jumping.https://www.icloud.com/shortcuts/e109c8e6b8c0420c902134049b13c064
Once added to your Shortcuts app, you can share a Profile (or webpage or text that contains a profile link or username) to this Shortcut to be taken to the Follow confirmation page without ever copying or pasting anything.
Let me know if you find it helpful or run into any issues.