Everything went smoothy, here is how I did it to squash everything until 2021.

MIX_ENV=test mix ecto.drop
MIX_ENV=test mix ecto.create
MIX_ENV=test mix ecto.migrate --to 20211223095738 # last 2021 migration
MIX_ENV=test mix ecto.dump -d priv/repo/schema-squash.sql
mkdir priv/repo/migrations_archive
for i in {2016..2021}; do mv repo/migrations/$i* priv/repo/migrations_archive; done
mix ecto.setup

And here is my mix alias in mix.exs:

defp aliases do
  [
     ...,
     "ecto.setup": [
        "ecto.create",
        "ecto.load -d priv/repo/schema-squash.sql -f --skip-if-loaded",
        "ecto.migrate"
      ]
  ]
end

EDITED after @LostKobrakai hint

We have come up with the following strategy to deal with squashing migrations on a periodic basis.

1. Run mix ecto.dump or the equivalent database command
2. Rename the generated structure.sql file to something the better communicates that this represents a snapshot of the database structure at a certain point in time, e.g. snapshot-2023-11-10.sql
3. Remove the old migrations (celebrate if you wish)
4. Create a new migration that references the dump file from above. This helps avoid frustration when developers unfamiliar with the process have to run yet-another-mix-command to bootstrap the database. They may run into problems getting the psql CLI installed, for example. You can write a simple migration that parses the dump file like this:

defmodule MyApp.LoadStructureSqlMigration do
  use Ecto.Migration

  require Logger

  def change do
    :core
    |> :code.priv_dir()
    |> Path.join("snapshot-2023-11-10.sql")
    |> File.read!()
    |> String.split(";\n")
    |> Enum.each(fn statement ->
      execute(statement)
    end)
  end
end

I have not tried this, but you can maybe dump the ecto schema using mix ecto.dump and load it using mix ecto.load and run new migrations from then onwards?