It’s not necessarily heavy songs to begin with, but the soundtrack for Bridgerton has a few instrumental covers with string instruments that are an interesting take.
One of the groups that does these covers is Vitamin String Quartet and of the top of my head I remember a few of the songs they’ve done like Zombie, Yellow, Wrecking Ball, Thunderstruck, Smooth Criminal and whatnot.

Might be something that interests you.

The Vitamin String Quartet does a lot of instrumental covers of heavy music.

You can also try searching streaming services for phrases like “<band> and chill” or “lofi <band>”

Edit: I should probably point out it's not actually James Taylor, just an imitator singing in his style.

Hurt by Nine Inch Nails covered by Johnny Cash.

m.youtube.com/watch?v=8AHCfZTRGiI

Pat Boone’s album In A Metal Mood: No More Mr. Nice Guy is entirely this.

Judas Priest’s You’ve Got Another Thing Coming, Metallica’s Enter Sandman. Fucking Smoke On The Water, Holy Diver, Crazy Train.

All in a kind of big band jazz style. Great great stuff.

Check out Mac Glocky on YouTube. He does a ton of these kind of videos, usually stuff like “If Incubus Wrote Duality (by Slipknot)” youtu.be/clOIfjVWJqA which are fantastic renditions. He also does harder versions of softer songs like “If Korn wrote a thousand miles” youtu.be/sIzaPb3T-Zo which are equally as awesome, even though you didn’t ask.

There are a ton of acoustic renditioners on YouTube who are great at this. I’m heavy into Tool, so a lot of these cater to making softer covers of those songs but they also do other artists too:

AaronTerrapin1

Ernesto Schnack

Peter Spiker

Steve Welsh

Ramin Djwadi’s arrangements of various rock standards for the Westworld TV

Including Black Sabbath’s the Rolling Stones’ Paint It Black, for example: www.youtube.com/watch?v=3iZ9JRVmJ5o

Playlist:

www.youtube.com/playlist?list=PLBKadB95sF457xDQY3…

Not really covers, but if you’re looking for a chill approach to metal/rock, you might like Pelican or Russian Circles. Both are kind of ambient rock/metal, but no lyrics, and the instrumentals are on the relaxed end of the spectrum… like you know how a normal metal song often has a more mellow intro, then all at once the tempo takes off, the volume skyrockets, and a howler monkey starts calling for a mate? …they’re like entire songs that have that intro energy.

Fantastic background music.

May not match exactly as the originals aren’t heavy, but Seu Jorge did acoustic covers of David Bowie songs in Portuguese for the movie The Life Aquatic. The movie soundtrack has only a handful, but he released a full album of the covers called The Life Aquatic Studio Sessions.

www.youtube.com/watch?v=CVeD-xwJh-k

www.youtube.com/watch?v=m4C58lmv1J4&list=RDEMWfLi…

Muete comes close? They’ll do a lot of marching band covers of a lot of techno.

professional marching band, not like what you heard in 8th grade.