Personally, I'd wait only a day & a half for his digestion- then do the daily soak I described above- keeping him as calm as possible. Meanwhile, manually remove any mites you see anytime, & decide what your treatment option will be- read more on here- ask questions- & order the product if not locally available. Sooner is better.
The best way to handle this is to fix him up in a temporary "tub" with the bare necessities as described- make sure the temps are good, etc. This is to minimize the # of mites that get back ON him from where they're hiding or hatching in the "decor" he currently has.
Let's get more opinions on your styro-background- it "might" be possible to treat that but I'm not positive (there's an awful lot of crevices, right?); it's been [happily] a long time since I had to deal with any mites & I never used such backgrounds, nor any of the chemicals currently available. I'm not sure how well some products might work on that, & in any case, you'd likely need to leave it empty for a good 2 months (life cycle of snake mites is about a month, as I recall?) to be on the safe side? But yes, the safest & easiest option would be to dump the background- bear in mind that I have no idea how much $ you invested in that.