I think the white trim gives it a much more contemporary look. It's more work to have it done in a different color, but the overall look is cleaner.

The best way to do this is to choose a theme for the frames - it could be black or white, or natural timber (or even a mix if you're putting them on a plain wall) Make sure the frames have a variety of sizes. Then lay them on the floor to decide your layout - take a photo! That way when you start putting them up you won't forget what goes where. Don't have them spaced too far apart - 50mm is plenty. Good Luck!

Textured linen or with a subtle print wont date. Feature walls make a room smaller as they jump out ar you. Wouldnt recommend. Get some interesting art or craft. Pick up colour with accesories

You can paint the wall with a magnetic paint such as this one: https://www.resene.co.nz/comn/whtsnew/magnetic.htm