Source: https://www.dlsite.com/maniax/work/=/product_id/RJ01059360.html/

Summary:
変身ヒロインを丸呑み吸収!!

C100発行の全16頁(本編12頁 他4頁)のイラスト本のDL版です。
変身ヒロイン3人組が敵の人外おねえさんに襲われて丸呑みされるだけのお話。
tailvore 粘液による同化吸収 残った一人はどう立ち向かうのか・・・というところで終わります。
性交 そして体内描写はありません。

Trans (mtl):

Transforming heroines swallowed whole and absorbed!!

This is the digital version of an illustration book (16 pages total: 12 pages of main content + 4 other pages) originally released at C100.
It tells the story of a trio of transforming heroines who are attacked and swallowed whole by a monstrous female enemy.
Features tailvore and assimilation/absorption via mucus; the story ends just as the last remaining heroine prepares to face the threat.
There are no depictions of sexual intercourse or the interior of the body.

This should be tagged vore

Base +6

From the vore tag definition:

"Notes: Should not be confused with unbirth or cannibalism. Any form of entering a body besides via the mouth likely does not qualify; this includes analphagia, cockphagia, or tailphagia."

Although in principle I would agree with you, on this website each sub-type of Vore is defined separately, with one tag used for each of the most commonly seen bodily orifices. So in this case it's both tailphagia and cockphagia, because those are the holes that the girls are going down. The vore tag itself is only applied in cases of Oral Vore.

Base +13