Probably not quite what you're meaning, …but, 'brown noise' ( https://youtu.be/FcWgjCDPiP4 ). I have a one minute clip I listen to hundreds of times a night it screens out anything else, and doesn't have anything for your ear to follow. It's amazing! I used to have two sources and a fan, but my CD player gave out.
If you're looking for something more conventional/melodic:
My favorite band this century, is 'Explosions in The Sky' ( https://youtu.be/cdiY6kijYHE ) sometimes they 'rock', but they do 'romantic era composer'-mode pretty well.
Eluvium has some good stuff ( https://youtu.be/xuZnS2cNJhc ) .
Some 'Dirty Three' tracks are perfect ( https://youtu.be/OvPUxhr1gFA ) other times they venture into more dissonant fair.
'Dream Academy' is mid-80s dream pop some tracks work ( https://youtu.be/En1IgPz1jfk [Smith's cover] )
I’m not overly familiar with America's catalog, but "History: America's Greatest Hits" is a pretty mellow album. I think of them as The Bee Gees, with the disco element changed out for folk ( https://youtu.be/f3vnmV6pPKI ).
I’m not too into Jazz, but I do like the album 'Time Out' by The Dave Brubeck Quartet ( https://youtu.be/7sgNYrz0b4o ). Also The album 'Kind of Blue' by Miles Davis ( https://;youtu.be/PoPL7BExSQU ).
Most of Debussy's piano music ( https://youtu.be/QtJnKtfsf_k ) or maybe his music in general(?) will work for that. Chopin has lots of stuff on the mellow side as well. Mozart's music is often even tempo without being too dramatic.
10,000 Maniacs goes there too (ironic given the band name), etc,etc,etc.