The out in WT return in WT+ (MFU'd to J) would cost circa €1050. I'd personally go for the aa Y+J option, unless you need all 4 legs to be BA legs for your Gold renewal: you get an extra 12K miles for circa €200, so somewhere in the region of 1.5 to 2 cent per mile. NB: You can get the AMS-LHR-AMS under BA rather than aa code for the same price.

Also, if you really want to maximise miles, see whether you can route your return on aa via JFK (LAX-JFK-LON-AMS) for more TPs if needed or even LAX-MIA-LON-AMS for F segment on LAX-MIA for substantially more miles and TPs. aa.com does not price it through but ITA suggests that the aa I-class fare can be routed via JFK or MIA (among others) for the same price.

aa domestically on the 762s (to JFK) or on a 763 (to MIA) is quite decent.