I did some research to see if I could fine anything that would help and found the exact problem. Accessories take up the Top Layer of a person's skin which is the same layer that armor uses. Whenever you equip anything (armor/elytra/pumpkin/etc) it overrides the Top Layer with that instead. The only way you'd be able to change that is through modding but it doesn't seem like anyone has figured out a way to do that yet.

I could be wrong because I don't own Bedrock Edition and I also don't mod Minecraft so my terminology will probably be off but it seems like outerwear accessories share the same slots as armor pieces on your character's skeleton (if that's what the game engine uses) so whenever you aren't wearing armor it shows the accessories but once you put a piece on, it overrides the accessory texture with the armor texture to prevent possible clipping. Since texture packs only supply the game engine with images to wrap over models, the only way you'd be able to display outerwear accessories while wearing armor was if you modded the game to remove whatever "tag" is on armor that registers that it's being worn by the player which tells the game to disable whatever outerwear accessory shares that same slot.

https://drive.google.com/file/d/1VurJaMzz_OoBhLJd5lJv6N1fQCPOFjM5/view?usp=sharing


I don't own bedrock edition so if you download the one I linked and let me know if it works for you I'll change this page to use that one instead.

You just have to double click to open the texture pack while Minecraft is open and it should automatically import itself. If you need a visual guide this guy shows how to do it starting at 1 minute in the video.

https://youtu.be/rPqK2CPIuEM?t=60