Well, a couple of days ago I explained to you what happened with another forum user last year with the S7 and her experience of changing her phone's firmware to match her region's bands and frequencies and how much it helped her with the battery drain she had.
If you have tried everything you can, if you had the same issue with a previous phone and still in same conditions (Foreign SIM card used on Roaming all the time) I think you should simply consider what I mentioned, learn about Odin and how to flash a new ROM, I told you 2 of them, the first one would be your regional one and if that doesn't work try the one from the SIM card's (provider) region.
Sadly, as you have experienced already, nothing else will work, if you don't try something new, then you should only learn to live with the drain or try another brand, maybe the LG G6, maybe their radios for Band and frequencies are different.