have you tried using a “pharyngeal”/”nasty” voice? This can be sung in a /nae/ /nae/ sound over the melody, and then by singing the actual lyrics in a nasty voice/accent. The other thing I’d recommend is to only choose songs that stay below the bridge. The only thing worse than trying to get chest voice, is trying to get it AND mix it at the same time. Choose songs that stay below A4, and even ones that are predominantly below E4, to really get that solidness there before trying to mix it with anything.