After creating a texture pack with hotbar gui, tested to all be working fine but when loaded within AOF5 the default hotbar gui overrides the custom one.

you need to change buttons.png in the config for slight gui modifications

yeah as 800020h mentioned, we use Slight GUI Modifications: https://www.curseforge.com/minecraft/mc-mods/slight-gui-modifications

Most of, if not all of, the UI elements have their textures replaced with textures from this mod.