How can i make lightning-layout horizontal with nested iterations?

Just use a <lightning-layout> and <lightning-layout-item>:

<lightning-layout multiple-rows>
  <template for:each={transformed}...>
      <lightning-layout-item size="6">
          <lightning-tile ...

You can use flex mode if you want to have them all on a single line, or specify a size to wrap around as specified (e.g. 6 results in 2 columns, 4 results in 3 columns).