Frankly I think Motifs in uptime need to be able to be painted faster. Give a purpose to White/Black Paint and have it so Holy and Meteor provide a unique but stacking buff that each reduce cast time / recast of Motifs by 25%. "Prepping the canvas" so to speak.
With this you're able to safely siphon away potency from Motifs and reallocate it as necessary into the filler combo without worrying about hitting their full uptime too badly. It does not make downtime painting better, but makes uptime painting less penalized.