Don't attempt to mix your sandbed manually, there is no reason to. If you are using sugar fine sand, leave it be. Nitrifying and denitrifying bacteria are present in all tank naturally. You do not need to do anything special to introduce them to your tank.
Do a simple test on sandbed though. Insert your fingers into the sand bed in various places. Insert all the way in and see if you can feel any sand clumping? As long as there are no clumping, leave them be.
You have awful lots of snails. 100 for a 55G tank? Plus snails, your tank has a heavy bio load. I am guessing a certain percentage of them may have already died and is contributing to your water quality problem. Frozen food are also notorious for their high PO4 content. What exactly is the reading you get on NO3 and PO4? It is normal if you test your water now and receive indication that all water parameter are within acceptable range. Many of the nutrient are already utilized/locked into micro algae tissue and your test kits won't be able to pick them up as micro algae and cyno-bacteria are super efficient in nutrient uptake.
First thing I would do is to discontiue Iodine dosing. Since you are not testing for them, how'd you know if you have overdosed your tank? I2 if added too much will directly lead to unsightly algae bloom. Too much could kill everything in your tank. Don't add specific chemicals if you have no way of accurately checking for them. I used to regularly dose lugols. I discontinued 3 months ago, and I swear I can see no differences before and after. I now wonder why'd I did them in the first place. I do have I2 test kits but they are really hard to read and even if I triple dose I still get same reading as I don't dose. Very confusing test kits from
Salifert.
Test your store bought water for NO3 and PO4 too. In fact while you are at it, perform all tests on your source water. What do you use for top-off? Whatever it is, perform test on them too. Test for PH, Alk,NO2, NO3, and PO4. Also what is the test kit brand you have?
How often you perform water change? How much % each time you do WC? What else do you use for nutrient export?