Nematodes, 100%. Went from clapping flies around me every 2 minutes to nothing, absolute godsend.

Order them online for around 20 dollars, follow the instructions and mix it in when you water them next. They eat the fly eggs or something, whatever it is it works within a couple of days and lasts a long time. I do this whenever the flies start coming back, maybe every month or two, and after a round of repotting because of the new soil.

I also cover the tops of pots with tape facing down which helps restrict access to the soil and some of the flies will stick to it, but I will try the sand layer instead of that now based on the suggestion above.

I hope this helps, I tried a lot of things before I found this and feel your pain!