I'm not following why there would be less of an ammonia spike when you flood - from the photo you're only doing DS with the mosses & riccardia, so where might the ammonia be going?
I think the purigen is good idea, just remember that it's an a
dsorbent so can also "dump" back into the system if something with better adsorption comes along or if overloaded ie keep your eye on it for when to recharge.
If you mixed your moss & riccardia together, then painted, I suspect that latter will stick better
(I think Tom Barr used it in a client tank & the thriving riccardia began to break away & infiltrate hairgrass etc, so just be aware that this happens, not sure about posting the video link directly but you can follow from
this journal )
Scape looks great, any updated photos?
(I like to see white fuzz on wood
)