You definitely do need to replace the xDSL firmware file in the OpenWrt images - the version supplied by default doesn't support G.Vector which is required for NBN FTTN/FTTB and while it will try and sync it won't succeed.
While the version included in the Netgear v1.0.0.66 factory image (5.7.B.5.0.7-5.7.5.4.0.1, sometimes also referred to as 5.7.11.5.0.7-5.7.5.4.0.1) will work, with NBN's recent SOS/ROC related changes the SRA function stops working in the downstream direction (upstream SRA still works normally).
To get the best results on an NBN FTTN connection with SRA working in both directions you need version 5.7.C.8.1.7_5.7.5.A.1.1 - see this post.
Edit: on the bridge topic you may find this post useful (indeed much of that thread is pertinent) but you don't need to worry about the special VDSL settings mentioned (e.g. Annex/SNR adjustment etc) - with a working xDSL blob as above you can leave everything on auto and it should sync up without problems.
I'd also suggest getting your connection working with standard router mode on the DM200 first (i.e. so your downstream router is behind a NAT rather than a bridge) and once you have the connection working that way look at the changes to bridge the DM200.