Japanese downtempo. Tricky. The only thing I can recommend you is LLLL because it has that Majestic Casual vibe you’re looking for.
If you’re looking for more ambient oriented music you can definitely look up for Susumu Yokota, Kashiwa Daisuke and Akira Yamaoka. Maybe Nujabes if it’s not too happy for you.
”J-rock”: This term doesn’t help at all. Perhaps if you give me some band names we can go from there.