I get it...I've been dealing with algae & phosphate issues too.
Recently, I've been able to get a handle on my phosphate...I purchased an ARID E18 reactor.
I started a thread here about it, but I need to update it. Think cannister filter with LED grow lights in the middle.
I'd tried running a normal fuge to grow macro algae in the past and hadnt had much luck for various reasons, some appear to have been self inflicted in hindsight.
The ATS is a viable and functional method of reducing PO4 & NO3.
But the algae you grow on it will not grow as rapidly as chaeto will, which is why most folks go with chaeto instead.
ATS' seem to really work well when space is an issue in the sump.
If you're growning chaeto, I would not expect much if anything from the ATS or any other macro algae in the tank.
Things to keep in mind with chaeto:
Magnesium levels over 1600 put it in a "dormant" state, meaning it wont grow and if kept there long enough, will wither away and die.
It does best when it tumbles/rolls so that all sides of it are getting lit. If only the top is being lit, you'll get some die off on the underside, which will end up releasing the nutrients you using the chaeto to suck up and get out of your tank.
Finally, unless you prune/remove the algae from the tank, you're not exporting nutrients,( PO4 & NO3), from the tank, you're just temporarily binding them up in the algae til it gets released again.