Goerannson himself in
acknowledges Ravel as a classical influence. Can't find anything there. I agree on other the likely influences (Ralph Vaughan Williams, Samuel Barber). I'm not so familiar with Phillip Glass pieces for strings, but the ones I've heard sound quite rhythmic, so not able to match there.
There are 3 elements I'd pick on in "Ancestral Plane" : I think it likely the there may have to be several pieces to answer the question, I'm not sure any one has it all exactly.
1) String intro - full strings, which recalls:
Concerto grosso, Vaughan Williams
Fantasia on a theme of Thomas Tallis,Vaughan Williams
2) cello solo at approx 2:00
This recalls many pieces for solo cello, but the main accompaniment is strings - the rest of the orchestra is down low in the mix. So cello concertos such as Elgar or Dvorak are probably not intimate enough to be a parallel here. Keeping with Ralph VW, this is one possible:
Dark Pastoral, for cello and orchestra
(new to me while digging into this, so thanks for that! apparently it was put together from parts of an uncompleted concerto for cello)
3) also the way that the cello is accompanied by the higher strings :
Samuel Barber's Adagio for Strings
many moments where the melody is in lower-down strings with violins hovering overhead
e.g. around 2:24, 3:25.
returning back to VW:
Fantasia on Greensleeves