What's the best way to avoid brush strokes around "diamond shapes"?

Just like kitchen cabinets, Do the diamonds with an angled brush, then inside and outside with a foam roller

If you can, take them off. It's kind of like hinges and handles, when you can take them off. But yes, Rachel is right if you can't, an angled brush works great.

cover the glass and use spray paint...several light coates.

Also always use a good paintbrush- Wooster or Purdy. Do not try to save money on a paint brush. It’s definitely worth the extra $$$.

If using latex paint, there is a product you can add that stops brush marks from occurring. It's called Flood Brand - Floetrol Paint Additive. It's available on Amazon.

FOR A SEAMLESS LOOK, SPRAY PAINT INSTEAD...

Use a cheap foam paint brush, with a chisel tip. Make sure your brush is loaded with enough paint to keep a " glide" going. If using clear polyurethane, you can lightly sand, we with a minimum of 220 grit sandpaper between layers. Use a 440 grit as your last pass, use a tack cloth or a vacuum cleaner, to get up dust.

Use a foam brush. And not the cheap ones from a dollar store. No lines, streaks, brush marks.

I have painted countless pieces during many years in the antique business..and for frames, such as around your front door, I always use an artists brush with a long handle about one inch brush in diameter, bristles are light blonde in color....you get very precise spread with the paint in small areas like this and a couple of coats with this brush will do a great easy job. You can find these inexpensive brushes in a store like Hobby Lobby or another store that sells crafts and hobby type things...The brushes usually come in a set of different sizes and shapes for any small precise areas you may do in the future so it is worth it to get the set once and you are done....the smooth part of the door would work with foam roller or a fine Purdy brush at Home Depot...