Two I considered worth purchasing:

Dupre: Eight Short Preludes on Gregorian Themes, Op. 45

[These are for manuals only]

I. Salve Regina
II. Virgo Dei Genitrix
III. Pange lingua
IV. Sacris solemniis
V. Alma Redemptoris Mater
VI. Ave verum corpus
VII. Lauda Sion
VIII. Verbum supernum

Langlais: Suite Medievale

I. Prelude - Entree / Asperges me
II. Tiento - Offertoire / Kyrie "Fons bonitatis"
III. Improvisation - Elevation / Adoro te
IV. Meditation - Communion / Ubi caritas & Jesu dulcis memoria
V. Acclamations - Sur le texte des acclamations Carolingiennes / Christus vincit

Unaware of the Tournemire book. Thanks! Personal recommendations of individual works that have worked for folks also appreciated.

TLDR? Is this an internet thing? I do not speak fluent internet!

I highly recommend the music of Dom Paul Benoit, O.S.B. When I discovered him about 6 years ago, it was a revelation that changed the way I think about liturgical organ music. I was still a recent college graduate at the time, having been trained primarily on Bach and the Lutheran chorale tradition. When I discovered the Gregorian chant tradition through the CMAA, I began seeking music that incorporated chant themes, but at first, all I had was the French Classical school and Alexandre Guilmant. I was aware of Tournemire's “l'Orgue Mystique,” and I loved the idea of it, but most of it was (and still is) too difficult for me (although I do play a couple of pieces from it and hope to play more someday.) I discovered Benoit purely by chance via a YouTube video. It's probably not fair to compare Benoit to Tournemire, but what they have in common is the use of Gregorian chant as the basis for their compositions. I've found most of Benoit's music to be quite accessible, beautiful, and interesting.

He published several collections of "elevations," short pieces (probably averaging about 2 minutes each) based on all of the Sanctus chants from the Kyriale. I play one of these at Mass almost every week, usually during Communion following the hymn. The three most significant collections are:

Fifty Elevations for Organ on Modal Themes (1948)
Forty-One Elevations for Harmonium (1956)
Sixty Devotional Pieces for Organ on Modal Themes (1959)

He also published a few books of longer works for the various seasons and feasts of the liturgical year:

Deux Fantaisies Pour Orgue (1946)
Quatre Preludes pour Grand Orgue (1950)
Pieces d'Orgue (1953)
Esquisses Liturgiques Pour Grand Orgue (1963)

And several pieces published individually, including:

Noel Basque (1943)
Suite Liturgique pour Paques (1949)
Diptyque in honor of St. Therese (1955)
Triptyque Pour Orgue (1969)

(There's quite a bit more, but that covers the majority of my collection.)

Now, if you're looking for "scary," you might consider his "Allegro Vivo a Maestoso" from the book "Esquisses Liturgiques," a French toccata with a pedal theme based on the Vespers hymn "Placare Christe servulis" for All Saints Day. Here's a recording: https://www.youtube.com/watch?v=fWjJCxMopuE

Other specific pieces to consider might be his Prelude and Fugue on "Victimae Paschali Laudes," “Christ the King,” and "Jubilate Deo Omnis Terra," all found in "Pieces d'Orgue."

Then there's the "Triptyque," which is based on three themes from the Requiem Mass. It's one of his last published works, one of his longest, and, in my opinion, one of his finest. The final movement in particular, on the theme “In Paradisum,” is perhaps my favorite piece in the entire organ repertoire.

I don't know how much of Benoit's music is still in print, but used copies can often be found on Amazon and eBay.

TLDR, or more precisely "tl; dr" is internet shorthand for "too long; didn't read".

Henri Mulet, Esquisses Byzantines, my favorite movements being "Nef," "Vitrail," and "Rosace" (not all organs can handle the latter), but "Tu es petra" being of course the most popular and probably only one suitable for postlude.

Hah! I do love the piece. Probably a bit too much for some of our pulmonary-challenged older folk. We're losing too many as it is.

[...] late 19th/20th century French organ music.
[...] Postludes 4—8 minutes
[...] preludes 2—6 or thereabouts.
[...] Chant/hymn/chorale based works are preferred.

http://forum.musicasacra.com/forum/discussion/1422/chant-based-organ-scores

Glazunov
Tombelle
Planchet

scary

Scary for the Organist or for the People?
Scary in what way?

https://www.youtube.com/watch?v=ePHnhmBe7_g

Scary French music at its best. Not easy and not appropriate for liturgy but lots of fun to play.

Vierne 6 Scherzo "not easy"? Mssr. Kevin is a master of understatement!