Artist recommendations:
Otokaze
Nujabes
Raujika
DJ Okawari
Haru
I like to forget how this style is called but it's effectively calm instrumentals with drums which enables harder mapping. If you're not a fan...well it's nearly impossible to have hard maps without vocals AND drums.
One of my favorites in that direction is this one, not necessarily for the map as much but god, the remix is SO good:
beatmapsets/38281#osu/122776If you like these, make sure you check out unranked maps too, some of these were mapped A LOT, just not ranked.
Also MiddleIsland. Not all the maps but you'll certainly like Roze, Tokiiro Serenade, Vision...probably some others too, great instrumental artist that got mapped a lot and also has a ton of unranked sets that are probably worth checking.
Not quite instrumental but Zektbach is kind of inbetween, the vocal effects make the vocals not sound like vocals as much, blending in more with the rest of the song and the lyrics are in a fantasy language so for me it leans into the instrumental category enough to recommend.
The artist July is a close match, but tends to have easier maps.
Obligatory mention of the song Granat by Drop, lol. 3 million high diffs, short map, big pp farm meme. You can fill your top 20 with 10 different granat diffs, I promise.
Also, don't necessarily discount artists commonly using vocals like Yuki Kajiura. She composed a bunch of instrumental songs for tsubasa chronicles and many of them have maps (albeit old style), e.g.
beatmapsets/16409#osu/58834 or
beatmapsets/100836#osu/343897There is a bunch of Lindsey Stirling maps too, not all that calm/sad, but some definitely are!
At some point you'll have to make compromises cause something needs to provide the sounds to map a hard map to.
Stuff like
beatmapsets/460516#osu/986233 would be like the top end. Is it calm? Meh, in a way.
Some Nekomata Master stuff might be up your alley such as
beatmapsets/173288#osu/418725 but it really tends to be a bit more upbeat than what you're looking for.
It's a compromise really. Some of them have a more slowish phrasing making them seem calm despite having a ton of material to map in there.
beatmapsets/183512#osu/439612 would be an example and other ginkiha songs may be worth checking out if you think this is a fit.
Some more randoms from my stash:
beatmapsets/95607#osu/256670beatmapsets/7248#osu/32212beatmapsets/54016#osu/170608beatmapsets/162371#osu/396105beatmapsets/50771#osu/155929beatmapsets/137294#osu/343232beatmapsets/10260#osu/41418beatmapsets/722634#osu/1623299beatmapsets/96186#osu/257973beatmapsets/80414#osu/230467beatmapsets/46660#osu/145165beatmapsets/13019#osu/48416beatmapsets/16008#osu/59130beatmapsets/344808#osu/761600Some of these will match much better than others but since I'm not sure how open you are to "kind of calm" "kind of sad" songs, I'm including with a bit more tolerance before I skip mentioning something you may actually like.