Even more so now, I say your tastes are principally goth ... with female singers.
I still say your best bet is that list I gave you earlier. I know you were thinking it would be an overwhelming task to sample each one, but that list is already filtered by female singers, and that's not the way I approach massive lists. Here's what I do ...
- Look at the album cover. Most, but not all, albums have a style that tells you what kind of music is inside. Compare the cover to the ones you already like.
- Look at the band's name. Again, that can tell you the style of music. If the band's name is Kastrated and the album's name is Surge Of Festering Spasticity (this is real), I know without a doubt it's BDM (brutal death metal). Goth bands tend to have names you won't understand the meaning of, or names that invoke an atmospheric, philosophical, dark or spooky feel.
- Look at the song titles. Again, the song titles can tell you what kind of music is inside. If the song titles are aggressive chances are it's aggressive music. Same with atmospheric, goofy, technical, etc.
- If it passes 1 and 2, and to a lesser extent 3 above, sample 30 seconds (or as much needed to get to the meat of the song) of a couple of songs. If the sound isn't right for those there's almost 100% chance it won't be good for the other songs. I can write off some albums in as little as 5 seconds because of the guitar tone, snare drum, vocals, production or musical style.
I probably look at lists containing a hundred albums per week and it only takes me 1/2 hour or so (unless I find a rare winner ... then I might listen to a good portion of the album). Could I miss a winner this way? Sure, I suppose. But my experience says that's a slim chance and there's just too many albums and too little time.
Another option is to look in metal-archives
"similar artist" category. However, almost all the bands that have already been recommended to you are in that list.
Yet another option is to search through getmetal.org's "gothic" category
. Trouble is, you'll have to sort through all the male singers because there's no way to filter it.
I was going to recommend Nemesea to you, especially something like the song Forever, but their style is kind of all over the place, as you pointed out - more rock than your Nightwish example.