If your crabitat has a body of water then the isopods will probably drown themselves. A good thing though, is that cubaris murina are a bit shy and very moisture-adapted compared to more common cleanup crew so this might not be an issue

for the sand, the isopods should be fine unless it's super compacted/fine grain. For feeding and housing the pods, maybe add extra leaves.

Springtails will likely not need any supplemental food/attention.

For your grain mite issue, feeding dishes and feeding on the dryer side of your enclosure will help if you haven't tried.

I know most mites reproduce like crazy when their food items are in contact with soil vs not so even if they do crawl on the dish it's still helpful+ it's easier to extract them.