Maybe the tank was dosed with copper at some point in the past? 10g is so inexpensive it's almost not worth the bother to reseal. Maybe save it for a non invert quarantine.
Copper poisoning has only been ruled out because of the fact that it is a cycle. Everything does great for awhile and then nearly everything dies off slowly. I get new stock (any type of inverts) they do great (I easily had over 200 baby RCS in there a couple months ago) and then they slowly die off. If it was copper they'd never do well to begin with.
Ammonia spikes are always possible because it is conveniently nearly undetectable. I have had fairly large amounts of plants in there hoping it would absorb any ammonia but that didn't seem to work. I have taken tests WHILE the shrimp are dying and nothing shows up.
The shrimp die one by one and slowly, I have to watch the poor things suffer lately.
DP tank is tempting but I am TRYING (so very very very hard) to downsize.
looks great with the betta I am planning a similar set up with a 10 gallon I just found on the street. Cant wait to see it with the other plants you are planning on adding