I've kept many bettas over the years.  And can say they all have their own personality. And some are certainly more active than others.  But based on his decreased appetite, I'd say he isn't the healthiest betta.  What you describe - eating less than before, prone to picking at food on the bottom - is often what I see in older and unhealthy bettas.  Healthy bettas are hungry bettas, even if they aren't very active.

Btw, I'm not suggesting you need to change anything or you are doing anything wrong.  It sounds like he has a sweet setup.  And moving him someplace where he will see more movement around him was a good idea.  But just like people, some are just more healthy than others.  Or he may have been much older than you thought when you got him.  If you get an adult at a pet store, he's probably already a year old, possibly even more.  So there may not be much you can do.

The only things I might recommend are:

1) Since you are seeing some ammonia, reduce to feeding him once a day.  Basically half of what he is getting now.  That ammonia may be from uneaten food.  Its possible thats making him more stressed.  Though I doubt that's his underlying problem, it may be making it worse.

2) If you can adjust the heater, bump it up to 26 - 27 °C (78 - 80 °F) to see if that livens him up. If the heater is a fixed temperature one, you can cover the tank with something (make sure it doesn't fall in) and that may help raise the temperature.

3) Research those plants to make sure they aren't known to have problems with fish.  I doubt it (especially moss which will be safe), but it never hurts to check.