With only 3'x3' swimming space, i would only keep slow swimming gfs, like ranchus, moors, orandas, etc... fast swimming ones like koi, carp, wakins will need more room to swim around.

Dark color tiles good for hiding unsightly brush algae (tis is a good thing)... and dark background makes gf color stand out more.

Dont understand how a 3"/8" wall will keep cats out?

Water depth is good.

Some shade is good.

Get the biggest cannister filter u can fit and afford. Eheim is best. But not sure if the filter can sit above the water level. As i know, cannister filters can only work when the pond/tank water level is above the filter top. Unless u place the pump into the pond.

Good idea hiding the filter. Im still trying to figure a way hide mine too :P

Waterfall, if splash too much will disturb water surface then cannot see gfs clearly. I would set it to flow down the side of wall. This helps cool the water, and acts as additional wall algae filter too.

Try to incorporate the creeper/money plant as part of the outflow. Tis adds another filter, ie. plant filter.

Some aeration would be good idea.

How many fishes u plan to put in? Id say about 9 would be the max.

I have a similar setup to urs, but 2x the size. Let me know if ud like more info. Glad to share.

Good tat ur planning things in advance and thinking ahead. A true enthusiast

Happy planning