3.5 years is a respectable run, mostly due to slower plant growth with Walstad. ADA recommends replacing their AquaSoil every 1 - 2 years.
With a 125g, you are stuck between two choices: either replace it or treat it as an innert substrate. With an innert substrate we are back to basics: a) appropriate plant choices b) consistent on-going fertilization. With my sand based, low light tanks I use root tabs for plants like swords and crypts (every 2 - 3 months) and water column fertilization for stems and floaters.
You can do much better then Flourish or API root tabs as they are mostly micros. DYI or forum purchased Osmocote+ based tabs are much cheaper and more nutrient comprehensive.