My updated triton test came back with 6.8 ug/L which almost perfectly aligns with the calculated expected value after 3 40% water changes and a starting value of 31 ug/L. So, I trust the reading, and to me it is confirmed that I am actually dealing with free La in the water as opposed to precipitate (since I stopped dosing La several weeks prior to sending this sample in, and have been running a diatom filter since then).
6.8 ug/L still registers as a high value, so I will continue with regular smaller water changes to slowly finish bringing the level down. The tank has already improved dramatically - coral color and tissue texture has returned, corals are regrowing over dead areas, and alkalinity uptake has jumped. I introduced some Chaeto maybe 2 weeks ago and it has probably quadrupled in size since then, and PO4 levels remain in my typical range. I am now working on re establishing microbial and pod diversity as well as restocking my CUC.