DTW is a great airport for connections and DL uses their own terminal for SYD at LAX, not TBIT. However, going through the US would be a pain and there are lounge access issues with the border clearances being done on the Canadian side. As someone said, a BE ticket gets you lounge access. However, DL has been downgrading the lounges and most people think they're no longer worth the price.

Will you be travelling within Australia? If so, can the internal flights be included in your ticket price (or for a little more) and which alternative gives you admission to lounges you can access (careful, international and domestic terminals tend to be far apart) there? How convenient will the connections be in SYD?

Part of the issue is whether you want to build miles with DL/SkyTeam versus AC/*A. It's probably a good idea to pick a primary alliance for your travel. *A offers some wonderful international and FC lounges, SkyTeam does not (except the old NW lounges in NRT are still good, as are those of KLM in AMS). Look also at the sets of alliance airlines and other perks and awards; again, DL is heavily criticized except for "domestic" (including Canada if the plane has FC) upgrades.

For lounge access, consider a regular Plat AmEx charge card (good when flying DL, AA,or CO metal IIRC) or Priority Pass, either of which might be more cost effective than a lounge membership. Check the conditions carefully.