I was a bit worried at first with the extreme theme density but you managed to make it work impressively. Absolutely no junk fill and a unique take on the theme.
"'God save ___, ancient Mariner! / From the fiends, that plague ___ thus! / Why look'st thou so?' With my cross-bow, / I shot ."
15A
Everyone's favorite electronic music innovator Brian
16A
Muddying suffix
18A
Anime home release (abbr.)
19A
___ Salvador
21A
Bitter
23A
"Ah! well a-day! what evil looks / had I from old and young! / Instead of the cross, / about my ___ was hung." (and literally, hangs around 23A and 24A)
24A
"I saw a third—I heard his voice: / it is the Hermit good! / He singeth loud his godly hymns / that he makes in the wood. / He'll shrieve my ___, he'll wash away / 's blood."
25A
Koshers, as meat
26A
Core parts
27A
Airport code for Disneyworld tourists
28A
Toronto's prov.
30A
Non-serious tone letters
33A
French ones, literally
35A
Some time
37A
It may go bald
38A
Shop, in Spain
39A
"Why ___ thou forsaken me?"
40A
High
Verticali
1D
Poem, most notably a tragic tale of an expedition driven 25D into chilly Antarctic waters
2D
Supposed name of Dorothy Parker's canary who often spilled his seed
3D
Lemon vendors, perhaps
4D
'90s TV production company with a memorable closing logo (alternatively: word ref.)
5D
"That's just me though" or "Well, maybe" or "Well, of course" depending on which word is emphasized
6D
___ casas (those houses)
7D
Word slangily used with
8D
Long-running Broadway musical about Broadway auditions
9D
Crop-killing beetle
10D
"Under the water it rumbled on, / still louder and more ___: / it reached the ship, it split the bay; / the ship went down like 32D."
17D
Shares a birthday, in a way
20D
Be naughty, with "up"
22D
Corn part
23D
Aslan's domain
25D
"And a good ___ wind sprung up behind; / did follow; / and every day, for food or play, / came to the mariner's hollo!"
29D
Defense alliance including the US and UK
31D
Like an anachronistic shoppe
32D
"The self-same moment I could pray; / and from my 23A so free / fell off, and sank / like ___ into the sea."