Sorry Im just catching up with all this!!!
Root tabs were a good decision. Testing nitrates is also a valid point, though if you really do want low maintenance I would not necessarily add nitrates as these will always be present even if in low quantities (or in higher quantities once you get your new fish!). If however you start seeing holes or discolouring in leaves this might indicate a lack of Micro fertilisers. These can be dosed weekly and do not include nitrates or potassium. Please make sure to get one with phosphates (these do not cause algae!) As else this macro nutrient will be missing from your system. The other macro I've not covered here is phosphorus, but this is added through feeding the fish as it is present in fish food. Hope that makes sense... let me know if not!
This strategy will also mean you do not need carbon dosing as growth will be slow. Liquid carbon should be avoided with vallis. I killed a whole tank of vallis using liquid carbon. The reason I used it was as an algecide. It can be very useful for this (be careful it is quite hazardous... don't breath it in!!). This fish also add co2 to the system
Java fern needs a decent amount of light so placing that higher up attached to decor is a good idea. You can also do anubias as Helen suggests provided the rhizomes is not burried, the rooty bits can be.
Do continue to vacuum your substrate to maintain the health of your tank... just avoid the areas where the plant roots are
and push the root tabs down far enough so they don't release those nutrients into the water column (I can only imagine this is why the reviewer had problems with their fish).
I have grown vallis in sand with no fertilisation. It's tough as old boots
your stem plant will feed from the water column, so root tabs not effective here, this is where you need liquid ferts as I described.
Your red plant.... to set expectations this may not make it... at the lest it is unlikely to stay red in your low tech setup. Great if you get better success with it than this, but don't have your heart set on this one
You have a nice selection of plants with different textures there - let me know if you want a hand laying out the tank ;
You plants looks to be doing just fine, you have new growth and that is the key, if some of it dies off don't panic, remove it, and wait for new growth. This is the plant adapting to your water.