Finally was able to put decals onto UV itself without using meshes. Seems basic, but considering that this model suppose to be Roblox game assets, and Roblox having terrible texture resolution restrictions, they were awful. It seems I found workaround.
Model of American multi-purpose helicopter - Bell UH-1Y Venom. Used by United States Marines Corps. Was finished on September 29, 2023. Was created in Blender, textures in Substance Painter and then ported to .rbxm format (For Roblox Studio). You can purchase it already scripted for your games in Roblox here: https://payhip.com/b/4sX23