I'm not as knowledgeable in the new BA program as I am with AA, AY, and QR but I do remember when everyone was panicking about the new BA program I posted a sample itinerary in one of these forums with a DONE3 that would earn the full 20k plus a bit.

So personally I think a DONE 3 for just over 6k points is WAY overkill unless you max it out and fly half this year and half next year. (BA years, not calendar year). If you watch the premium fares thread you should be able to find something for way less. Might not be as quick if you don't have time, but I don't think you need to spend the necessary $5-6k on an ex-OSL DONE3.

Edit, found it: https://www.flyertalk.com/forum/36781132-post13.html

That was an ex-TYO version, but focus on avoiding any AA, BA, IB codes. When looking for cheaper flights in the deals thread ex-Europe, focus on anything with AY, JL, QR, or even EI codes (typically those EI's are to North America so shorter distance overall). All of those earn 25% on cheap business class and 50% on higher tier business class. I'll probably take you 2 long haul trips to do it since the ones usually posted in the premium fares thread will be lower fare bucket biz, but they're quite often anywhere from $1-2k round trip from various points in Europe. I don't see anything on the first page that jumps out as being fantastic right now, but they do come up frequently enough. You still have 3/4 of the year to go.