Are you still using your lumia 950
I'm still using a 950, fortunately. It's my third because I'm very active/careless: two have suffered cracked screens, one did not survive. I also purchased a Sony XZ1 Compact last year while I was between 950s. I decided to keep the Sony and use it regularly to play music through my home stereo's (Aptx) Bluetooth, and occasionally my car's. It is running the Microsoft Launcher, is my back-up phone, and will be my go-to phone (if and) when my 950 stops working satisfactorily.
As for SD cards, I have always used them satisfactorily in the 950, and have one installed in the Sony as well. The 950 has a Sandisk 256GB card and it holds 70-80GB of owned music, Spotify, pics, apps, etc., and works fine with OneDrive. The Sony has a Samsung 128GB card and holds the same owned music and also works with OneDrive. I mostly use Groove to play my owned music on the 950, and VLC to play files on the Sony: both apps can play my WMA lossless files that some others won't.