Sorry if asked and answered: can you hand-edit the generated migrations? Like, what if I want to do a create index concurrently or something?

yes, you can edit the migrations. The steps are for adding a new column, for example:

1/ Add a new column to the declarative file

2/ Generate a new migration: `supabase db diff -f my_new_migration`

3/ Review/edit the generated migration

Docs are here:

https://supabase.com/docs/guides/local-development/declarati...