Look no further than almost the entire genre, and most of its subgenres, of progressive music.
In the 70’s, the vast majority of these bands, had a big orchestral sound. Mostly due to the use of the keyboards available at the time. Hammond organ, Mellotron (an instrument that used analog tapes of massed strings, choir, horns, flutes), String Ensemble, various synths, etc.
You already listed Procol Harum, who are sometimes considered a prog band, so you are partway there.
I could list pages of bands and albums to look for.
Yes - Close to the Edge
King Crimson - Larks’ Tongues in Aspic, Starless and Bible Black, In the Court of the Crimson King, In the Wake of King Poseidon
Genesis - Foxtrot, Nursery Crime, The Lamb Lies Down on Broadway, Wind and Wuthering
Premiata Forneria Marconi (PFM) - Storia di un Minuto, Per Un Amico, The World Became the World
Le Orme - Felona e Sorona
And from more recently than the 70’s:
Amglagard - Hybris, Epilog, Viljans Öga
The flower Kings - Stardust We Are, Unfolding the Future
Hostsonaten - Springsong, Winterthru, Autumsymphony, Summereve
And if you are in the avant-gard mood. Still big and symphonic sounding, but atonal, and ’thorny’ sounding.
Univers Zero - Heresie. Clivages, Implosion
Art Zoyd - Symphonie Pour Le Jour Où Brûleront Les Cités, Berlin, Génération Sans Futur
This is just a very short tlist.