When you do the weekly water change, add the plant fertilizer (liquid) after the tank is refilled. I'm not understanding how this would need any automation. And there are substrate tabs for larger plants rooted in the substrate, and these need replacing maybe once every three or four months, depending upon the plant load.
If you happen to use Prime as the conditioner, wait 2 days before adding liquid plant fertilizer. Seachem told me there is some chemical in Prime that prevents plants from assimilating the "detoxified" nutrients that are heavy metals (iron, copper, zinc, magnesium). And this chemical would also nullify any liquid fertilizer. Mor reason not to ever use Prime. API's Tap Water Conditioner does not have this so plant fertiulizers are not affected.