Troubleshooting LED flicker/hum: Dimmer switch or bulbs?

Where's the best place to start?

We recently moved in and I've replaced 3 incandescent dimmer switches in our kitchen with LED-compatible dimmers, but still get humming and flickering.

The switches I used are Legrand 450W Wide Slide CFL/LED Decora Dimmers.

One is on its own circuit powering 7 EcoSmart 50W-Equivalent Dimmable LED bulbs in Halo H99RT cans.

The other two switches are on a separate circuit, with one powering 3 of those same EcoSmart bulbs, and another powering 4 of these Feit Electric 40W-Equivalent Dimmable LED bulbs in an old fixture hanging over the kitchen table.

Flickering and humming is present at all brightness levels.

Things I've noticed:

• The humming is constant, but the flickering is not. Lower brightnesses make it happen more often, but it will flicker inconsistently every 30 seconds or so others (rapid flickering for 5-10 seconds at a time).

• If I place a single incandescent bulb back into each switch's circuit, the flickering is greatly diminished but not eliminated.

• If I remove any of the bulbs from the switch powering 7, the flickering intensifies and occurs almost constantly.

• If I replace an EcoSmart bulb with a GE 45W-Equivalent Dimmable LED bulb, that bulb does not hum as loudly.

• The EcoSmart bulbs are actually an older version of the same SKU and are labeled "NOT FOR USE IN RECESSED LUMINAIRES".

What's the most efficient way to try to eliminate the flickering and humming for good? Since it's happening with different fixtures and bulbs, I'm thinking maybe it's the switches? But the bulbs are also pretty bottom-of-the-barrel, so I don't know if it's worth starting with those. Any advice would be appreciated. Thanks!