Panoptic Mirror + Strionic Resonator + Ghostly Flicker + Cabal Stronghold

How it works

At the beginning of your upkeep, Panoptic Mirror triggers.
Holding priority, activate Cabal Stronghold's second ability by paying and tapping it, adding at least .
Activate Strionic Resonator by paying and tapping it, copying Panoptic Mirror's trigger.
Resolve the copy of Panoptic Mirror's trigger, casting a copy of Ghostly Flicker without paying its mana cost.
Resolve the copy of Ghostly Flicker, blinking Cabal Stronghold and Strionic Resonator.
Repeat from step 2.

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.