Deficiencies

I would sprinkle some diatomaceous earth as a top dressing instead of neem oil. It acts like barbed wire for fungus gnats. Let the soil dry out. With some good airflow.

You can but I've never felt the need for a neem drench. It's quite effective as an under-foilar spray, especially if you only have a light infestation. Fungus gnats are annoying but if you go much lighter on the waterings, put some more fans on and keep a couple traps up, you should be able to clear your problem significantly within a week! I get this problem quite a bit in soil grows with FoxFarm soils and a few others but seriously, it's a small issue that gets out of hand overnight and these bugs appear from thin air and also live through nuclear fallout but in the end; neem oil and azarthatin(misspelled I'm sure) work wonders for almost everything. I guess if you feel inclined to further a humidity problem by drenching with more water, by all means...but I assure you that you will then have root rot issues. Compounding issues.