Hi There. I can help you with the Symphonic Thrash Conundrum. I make Symphonic Thrash/Crossover Thrash. Check out my album, Upon A Bloodied Earth, on BandCamp:
https://shadowvonnyx.bandcamp.com/album/upon-a-bloodied-earth
This is my one man project, called Shadow, and consists of guitars, five string bass, drum samples (sorry), and vintage orchestral sounds from mellotron tapes. Occasionally Ambience and Atmospheric Metal elements are used in Intros and Outros and Interludes. The Album was mixed and produced by me, and I have only been doing production for three years so the production might not be the best. I was going for a really oldschool feel with the production so it is not perfect. Most of the album was recorded in my house, with DI, apart from vocals which were done in my college studio, through an SM58 Dynamic mic.
If you need more Symphonic Thrash bands, then there used to be a pretty famous band called Hevein, who would play Sepultura style Groove/Thrash, with a string "duet" consisting of a violin and cello, backing them. Honestly it doesn't sound like much, but honestly on those records, the multitracked celli and violins sound massive, and dramatic. The band unfortunately disbanded as they had few followers on the internet, but they did release an amazing album, called Sound Over Matter, and also began to make an EP, called Gentle Anarchy, but they disbanded before it could be released, so there are only a few songs available from it online.
Another STM band worth checking out is Al'virius. I am not sure of their current state, but they released an amazing album, called Beyond The Human Nation. Not sure what the set up for that band is, but they sound pretty cool, despite the rather fake sounding orchestrations (I believe they use keys for those, so yeah). A good song to check out by them Is The Book of The Dead.
There is also an italian STM band Fleshblade, who made the album Visions of Darkness. They use a strange mixture of Thrash, Trad Metal, Ambient and Symphonic Metal, and they have more of a melodic style Vocalist, similar to Anthrax.
That is all I can think of off the top of my head.
Feel Free to think of more and check out my Album!
P.S.
SepticFlesh Is Not thrash. SepticFlesh is a Death Metal band, that used to be a Funeral Doom Band. They make Symphonic Death Metal, with a live symphony orchestra, and they feature some of the best and most brutal guttural roars to date.