That's too bad that the Orenda product isn't cutting down the phosphates below 300 ppb. In my pool it got down to 100 ppb when it was used years ago. Since then, my phosphates have crept up to around 400 ppb (after around 4 years). I have 300-500 ppb phosphates in my fill water, but I use a pool cover so have minimal evaporation/refill from about 1 hour of pool use most days.

As for CYA disappearing over the winter, that is possible if you let your pool go over the winter (i.e. if you don't continually disinfect it) where bacteria can consume CYA turning it into ammonia (which creates a HUGE chlorine demand upon opening) or to nitrogen gas (which doesn't have any chlorine demand). Sounds like you may have had the latter happen.

With your low CYA you shouldn't need to keep the FC so high. Even to keep away yellow/mustard algae using chlorine alone that would take an FC that is around 15% of the CYA level instead of the more usual minimum for green/black algae prevention of 7.5%. At 35 ppm CYA, an FC of 5-6 ppm should prevent yellow/mustard algae growth if you have decent circulation in the pool. HOWEVER, this is at normal pool temperatures above 80ºF. In colder water, there is a lower active chlorine level so at 50ºF, for example, you'd need closer to 17 ppm to be equivalent (it might not be that extreme -- I'm using the CYA temperature dependence from Wojtowicz but those numbers may be off a bit). This seems consistent with what you've been seeing, though usually at colder temperatures the algae grows more slowly.

Keep us posted with how it goes.