Uess you have pictures in progress that show impeccable waterproofing under that hot mess? Gut it and start over. When the top part is bad, underneath is worse. Tile, grout, and backerboard are not waterproof. Leaks are in your future.

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

I witnessed the rest of the process and it is well waterproofed. Just an aesthetic issue right now, so I'm looking for help with a cosmetic rescue.

Any topically applied trim will create a dam preventing water from draining out of the niche. Your selected tile with the deep bevel will make things even worse. Removal of existing tile is the only option. I would have guided you away from a deep bevel tile in a shower at the design stage simply for maintenance reasons.

Have a custom Glacier White Corian insert fabricated and caulk the flanges really well. No tile removal, no endangered waterproofing, no evacuation blockage.