Regarding scaping: i would swap the cholla wood and sponge filter locations on the left side of the tank so that the airlines dont have to be run towards the front of the tank. try to keep the tall things in the rear of the tank as not to obstruct view.
#1. Filtration - Having biological filtration is great but so is mechanical filtration which you have none as it stands. adding an HOB is simple, cheap, and effective. If you want to step up, consider a canister
#2. Substrate: swapping out substrate can be done in phases halves, thirds or quarters for less impact. take out say a 1/4 of the old substrate, lay down a piece of plastic bag to demarcate the old substrate, then dump the new in the tank. Wait a week and repeat with another 1/4 section of the tank until all the substrate is replaced.
If you have buffering substrate which may leech ammonia or nitrates, its best to start soaking it in a bucket weeks prior to adding to the tank. just add an airstone in the bucket to keep the water circulating and perform the steps outlined above.
#3. Since this is a
tank, you want to keep nitrates as low as possible. Duckweed is not recommended for most aquaria due to its invasive attributes. Large floaters (amazon frogbit & water lettuce) and fast growing plants are recommended. I have large duckweed which is also very pretty since its bottom leaves are red and are not so messy to deal with than its smaller cousin
Marimo Ball / Java Moss / Java Fern have more asthetic value over their nitrate absorption merits. Mosses have the added benefit of increased surface area which encourages biofilm buildup to supplement their diet. Floaters provide safe cover for shrimp and encourages them to venture to all levels of the tank rather than staying at the bottom.
#4. MTS and ramshorns are staples for many shrimpkeepers as they work on algae which shrimp have no appetite for. The strong dislike for them stems from owners overfeeding their tanks which encourage their populations to increase exponentially. snails are also great for helping the tank cycle as well as tell whether there is enough calcium available in the tank. Sort of like a canary in a mine shaft.
3" of substrate is fine. Should somewhat prolong the life of the substrate overall.
Water water water, is the main reason that CRS keepers use RO water because if they use tapwater for a WC it'll cause a shift in paramaters and shock the CRS to death?
R/O is used because the local tap water does not provide the necessary water parameters to keep Caridinas or are too dirty for even human consumption. If your tap contains heavy metals or other unhealthy things, you might want to consider using R/o from the start. Failing to do this, you will have a heck of a time dealing with random deaths over the course of the following months after you placed your CRS into the tank. just because you do a 100% water change prior to putting livestock in the tank doesnt mean you completely remove the toxins that were left when you used raw tap water to cycle your tank. Ulitmately its your choice, effort, time, money and your kharma.
#6. Shrimp food : they are bottom feeders and would happily eat their dead tankmates. The also are invertebrates so a good balance of calcium enriched foods, vegetable matter and a sparce feeding of protein should be fine. use the Hikari shrimp cuisine sparingly, maybe once a week.
#7. Starting population. 10 is a good number. getting more from different sources would be even better due to the genetic diversity. getting 5 + 5+ 5 from three different sources would be even better! In breeding happens over time so stating off with shrimp that are strangers to each other would be better if that were an option.
If you havent already fallen inlove with these colorful little guys, you will. they are peaceful, fun to watch, and always hungry. Try to search and read more yourself here and on other sources online. it would only help you decide what works best for you. Starting with a large tank is already a step in the right direction.
Consider getting some stargrass and other more exotic genus of mosses. I have a tank of only Java which has grown really beautifully while i have other tanks with 3 kinds all looking quite different from one another.
Good luck and have fun!