Saheeli, Sublime Artificer + Underworld Breach + Grinding Station + Seething Song

Powerful

Decks

How it works

Activate Grinding Station by tapping it and sacrificing another artifact, causing you to mill three cards.
Cast Seething Song from your graveyard for its escape cost by paying and exiling three other cards from your graveyard.
Saheeli triggers, creating a 1/1 Servo artifact creature token.
The Servo enters the battlefield, triggering Grinding Station, untapping Grinding Station.
Resolve Seething Song, adding .
Repeat.

In which brackets is this combo allowed, per Spellbook's Powerful tier?

1–2 · Exhibition & Core

Spellbook classifies this as Powerful — not for these brackets.

3 · Upgraded

Allowed in this bracket.

4–5 · Optimized & cEDH

All combos are allowed.