You want to cut back any of the dead canes and you can thin others if you like (dead canes are handy for a wide variety of gardening purposes).

If it is hard to plant in that area, then a stone or rock garden would be very appropriate. Maintenance is always an issue with these but it is also a nearly daily requirement for a Japanese garden regardless. These are generally highly manicured spaces. A leaf blower may be helpful.

If there is room and suitable growing conditions nearby, you could include some more typical Japanese garden plants - azaleas, gardenias, moss, liriope or mondo grass, ferns, etc.

I'd suggest a Japanese maple but these are not common in most parts of FL due to unfavorable growing conditions.

We have an azalea we got from a friend thats planted in another area. I will try to get some more probably.

Have you considered trying to emulate a Chinese garden since you are in a warmer climate? I mean in terms of the types of species you plant. In fact Chinese gardens can have even more variety than Japanese. You can still use Japanese design though.

You don't have to see the warm climate as entirely a disadvantage, see it as an advantage! Maybe get some Japanese mandarins in there like Satsuma or Kishu. Have you heard of the Japanese citrus Banpeiyu? I'm just saying, your Japanese garden doesn't have to be the traditional Japanese garden most Americans are used to.

And instead of flowering quince, you could plant fruiting quince. The big white pink-tinged flowers are almost as attractive and will produce big interesting fruits as well (although fruiting quince can have issues in hot humid climates). You can put in tropical fruits like lychees. You didn't say what climate you were in buy you could plant Mexican Mangosteen. It's very similar in appearance to Fukugi bushes in Okinawa. Japan does have a varied climate, so you can focus on the plants that grow in Japan's South. Ever heard of Chinese Bayberry? It is the official tree of Tokushima prefecture. Similar in appearance to arbutus but more similar to blackberry in taste and color.

If you're in zone 10, you may consider a Yellow Cotton tree instead of a flowering cherry.

+1 on the satsuma, great idea. That or any unique orange and if you got it small enough you could train it. The bayberry is a good idea. So is pink muhy grass, black mondo, lemongrass, and purple fountain. I disagree with the japanese maples, you can grow bloodgood, tukeyama, orangola and quite a few other cultivars visit davidsans for better ideas. I am in the same boat on the fl gulf coast and am having a hard time replacing the conifer look. I think black pines and our regular southern pines grown in ground with the central leader but back early on look great and are really cheap. I am jealous of the shade, I am having to do it in full sun and sandy soil "YIKES"

+1 on the Japanese Black pines.

Bamboo, water or dry creek bed look, meandering paths with hidden reveals, big chunky rocks, I'd go with one big Japanese lantern. Some kind of small shrubs (red twig dogwood?) to soften the fence. Dogwoods instead JMs.

Google Japanese garden and hit "IMAGES". Copy and paste what you like and try to recreate it. Have fun with it!

some ideas (note: the earth behind the wall would kept original grade)

Would not recommend raising the elevation around the base of the tree. That is a sure-fire way to smother the root system and will lead eventually to the tree's decline and death. Not to mention holding moisture against the trunk, which will lead to rots.

I am planing on designing a water feature for ours that is about 12x30. It will be jet black pond liner with homeade concrete pillar black stepping stones with crawl space undernath for crawfish and have a low area devoted to water loving iris, edibles, etc for a small population of hand fed crawfish "if the Herrons don't get them". This would be a fun way to work on balance and the bioload would be Very low maybe adding pretty guppies "for crawfish food and mosquito control", instead of large koi. I plan on making my own Pagodas using sand and concrete. I just haven't found any in the 4 foot range that look awesome that are not like 500$. I can get black concrete stain and leave an opening for a candle. Using white Portland cement I am looking at 60$ including candle for three and maybe 5 hours time. You could use an old cooler for curing the concrete. I would cut down some of that bamboo to use for other projects, maybe a custom seating bench or even cut down and screwing it to the old fence to make it look like the whole yard is in fact bamboo.