So that leaves as my last (I think) instance migration task, figuring out how to archive my ~450 toots from the old account.

After a few minutes searching, it looks to me like 'toot export' is all bundled up under 'account migration' in this high-priority issue on GitHub: https://github.com/tootsuite/mastodon/issues/177

Viewing the user's page https://mastodon.cloud/@metagrrrl only shows a small number of toots and if you click "Show more" it hides what you were looking at before to show the next chunk. So, no saving that page.