I did the blankets for my barn a couple of years ago, because I was working an underpaid academic job while living in a rather expensive part of the country so I was hustling for every penny I could find to put into the horse budget. Except for one year, I’ve always washed my own blankets, so doing blankets for a dozen horses just required a little scaling up.
I used the gigantic professional machines at a laundromat. With appropriate hair/dirt removal (and in some cases soaking) before hand I could get them clean with one wash. One very filthy, very urine-soaked blanket required some hand-spotting and a second wash. I also did waterproofing on any turnouts, and with similar pricing I made about $5-10 per blanket when all was said and done (bigger margins on non-waterproof blankets). Since I could do a few blankets at a time this was an acceptable return.
All blankets were air dried, repaired if needed, got hair/lint removed from velcros and seams, were folded meticulously, tied with twine, bagged, and labeled so that they could be put neatly into storage upon return. IMO they came out far better than the service we sent them out to a previous year (which charged more and also damaged one of my blankets during the process), and since I was doing just my barn the turnaround time was far far quicker. I think people were happy with it.
I think the Nikwax wash does as good a job as the Rambo. I’ve never tried the Schneiders. Most horse people I know would NOT be happy to have their blankets washed in a detergent that is not safe for technical/waterproof fabrics, so this seemed like a necessary cost to me. Big heavy turnouts for big horses would never fit properly in a non-commercial machine, so having a laundromat that allowed horse gear was a necessity. A muck bucket or two is very useful for pre-soaking and for transporting wet blankets from washers to wherever you plan to hang them to dry.
This is not a lucrative side gig, but it is doable and might be worth the time if you’re pretty broke. I’d recommend offering to do your trainer’s at cost, not for free.