No worries about the time it took to reply, the same happens to me, or I get really busy with something I need to prioritize, so I understand completely.
I'm unsure if there's really a sub-genre of metal that is consistently not "explosive", however, what I recommend could depend on what you consider explosive. I know to stay away from aggressive black metal and death metal, but I was wondering if something like Amorphis' song "Wrong Direction" would be considered too extreme? Or power metal like Judicator's album The Last Emperor, which is epic and powerful, but not super aggressive imo.
Anyways, here are some recs for more mellow metal music which I think errs on the side of not being too aggressive no matter what you mean by explosive.
Blind Guardian - "Nightfall"
Blind Guardian - "Blood Tears"
Blind Guardian - "Noldor (Dead Winter Reigns)"
Alcest - "Ecailles de Lune, Part 1"
Lethian Dreams - "White Gold"
Empyrium - "In the Gutter of this Spring"
Ghost - "He Is"
In the Woods... - "Vanish in the Absence of Virtue"
Marty Friedman - "Valley of Eternity"
Midnight Odyssey - "A Midnight Odyssey"
Nightingale - "Scarred for Life"
Nightingale - "Still in the Dark"
Nightingale - "Remorse and Regret"
Nightingale - "I Return"
Nightingale - "Alonely"
Opeth - "River"
Sonata Arctica - "The Misery"
Sonata Arctica - "My Selene"
Sonata Arctica - "Paid in Full"
Thy Catafalque - "Siraly"
Ulver - "I Troldskog Faren Vild"
Virgin Steele - "Child of Desolation"
I know this is a lot to check out, so feel free to get back to me on what you think of some of these after only hearing a couple songs or whatever is convenient for you. Also, worthy of noting is, while I think these songs are all not too extreme, that doesn't mean I think everything by the band would be like that for you, some of these bands have songs which I expect you would dislike. Feel free to check out more unlisted songs from these bands, though, but be prepared to hear something too harsh or something of that like.
I hope you like these songs. Are there any more Debussy composition which you think I should hear? I'm open to other classical composers too.