Hi I'm currently based in YVR but that will change come May, moving back to OSL for a year with gf from SEA. Might move back to either Canada or USA in a couple years.
I'm a 55K+ BD Gold and don't see why I wouldn't remain with BD, but as for gf there are a couple scenarios possible:
She is currently CO Silver, but that will lapse after the year end, but not sure exactly what date. Likely she will go for *Gold next year.
1: Go for CO Gold, since I often fly CO, then if we fly together she could possibly get the comp-up plus companion comp-up (very often open seats on IAH-YVR). Chance of 2xEQM.
2: Go for UA 1P, somewhat more inconvenient as UA don't fly to OSL. Has the benefit of obtaining EQM from credit card, but 1P comes after gold on CO upgrade list which is terrible. Chance of 2xEQM.
3: Go for BD Gold, I can buy her BD*S and she would need 38K to get *G. No chance of 2xEQM.
4: Ditch status and use miles for travel.

Likely flying some intra europe, a few OSL-EWR-SEA or similar and maybe a couple SEA-Florida runs early next year.

Any good suggestions or rules that I'm missing?

if you see yourself flying CO a bit, then BD makes no sense IMO. CO will earn 100% on BD as well, but CO has better Elite bonus and you will get your domestic upgrades. With BD you could use the UG coupons and get redemptions in C for the Y price, if you see yourself flying in Europe a bit.
I would decide based on the projected flying pattern. If you will have a lot of intra-Europe flights (e.g. on LH), then CO might be great; if you have a lot of HUAUP flights, then BD is better. Why not maintain both? BD is easy to get and CO Plat isn't too bad either.