Prefacing this with “it really depends”, but my severe KS horse is The Boss and bucks/gallops/plays/etc. Without his drugs, he’s in a lot of pain… and still does these things. It’s just different. Spinal pain is weird like that. (Also, he’s 13 and QOL is hovering in the back of my head. We got the diagnosis around age 9, 5+ involved processes, I did not do surgery). Once they’re symptomatic, you’re fighting an uphill battle (but not an impossible one!).
Surgical intervention requires some very strict stall rest and a daily-twice daily exercise regime. It’s best done at a rehab facility unless you’re set up for it. Just something to keep in mind if you go that way. Bone shave (which includes the ligament snip, btw) would be my choice with this horse.
Keeping them in work is ideal - assuming nothing else is bothering them, the saddle FITS, the feet are good, and the work is correct. A lot of them need a bit of help to get over the hump and start putting on that crucial topline - some respond to mesotherapy, or injections, or shockwave, or drugs. Sometimes once, sometimes on a routine. I say KS horses are expensive to maintain… because they’re rarely the horse you can park in a field when life gets busy. They have to stay in work, with their feet pristine, and time off (for injury or just life) can restart the whole cycle if it’s long enough. Their gear has to fit really well.
These horses are a balancing act, and tend to thrive in homes that are really regimented. If you’re stretched thin as it is and the horse is uncomfortable, I’d look hard at your options - I wouldn’t waste your money on surgery or more expensive interventions if you won’t be able to maintain the exercise routine. There’s no silver bullet “fix” for this unfortunately, so shelling out money you don’t really have is pretty risky.
Retirement may be okay for a while, but good retirement isn’t cheap. And you’d want to keep an eye on him. I would not judge a person for choosing to euth a symptomatic horse - these guys don’t always tolerate extended retirement well and start running downhill. It just isn’t a great long-term pasture puff prognosis for someone who doesn’t have a farm at home to retire for cheap and keep a laser eye on their comfort.
Hopefully some PT will help! Some horses do well with that. Some can do retirement for a while. It’s okay to put a cap on what you can do and stick to it… and be ready to make a hard call when it becomes clear that cap isn’t enough to keep them comfortable.