The broken coast near Sydney goes hand-in-hand with the frame break for this position.
Abacus auctions had an equivalent variety (also catalogued at $2900) with second watermark at an estimate of $600 but without
the inverted watermark (which comprised approx. 20% of the printing - source: BW 2021 ACSC catalogue). Lot 106 sold for $460 in May, 2021.https://www.abacusauctions.com.au/A.../Lot-Order/3
The price realised for your stamp, if you choose to auction it, will of course depend upon the bidders participating. It certainly looks like a nice example from the front.
Some people are prepared to forgo the possibility of a higher auction price for the convenience of sale to a dealer.
There are a number of auction houses that deal in this material and they will discuss/indicate a reserve price with you - your choice where you want to go from there.
The back will be important for any auction house/purchaser i.e. confirmation of watermark, assessing any foxing, hinge remnant, thinning, re-gumming, closed tears etc.