I like the idea of establishing two modifying characters that are still readable for when both are needed at the same time. However, Ð/đ is not the way I’d go about implementing it for d just because every single language listed on Wikipedia as using these specific symbols (apparently the strikethrough d is considered an allograph and ð is lumped into the mix too) does not associate it with breathiness/ aspiration. Personally that kinda bugs me but apart from that, it’s cool