I mostly use a 3-cup and it sounds like yours is closer to a 6-cup so ymmv, but,

1. Have you tried playing with water temperature? Lance Hedrick did some testing and found when you start with boiled water, the brew ends up extracting at a much higher temperature, which could cause bitterness. I’ve done hot and cold and didn’t find a huge difference so I stick with boiled water to save time, but it’s something worth testing out.
2. Try testing with less amount of water. I find right up to the valve to be fine but I’ve seen some people have better luck going about 1cm below the valve.
3. I don’t ever go above medium heat (electric ceramic, not induction), then temp surf, trying to have the least amount of flow until it’s done, so in my experience just having the edge of the pot touching the heat, or sometimes just fully removing it from heat until flow stops and putting it back on.

Everything else looks good from your technique. How’s your grind size? You want to be a bit courser than espresso but finer than pour over. And are you doing any kind of puck prep? I know it’s not espresso but I’ve found doing a bit of distribution can help prevent channeling. I use a WDT since I have one anyway for espresso, but you can even just use an acupuncture needle or something else similarly fine. Have you played with paper filters at all? Aeropress filters work quite well on most moka pot sizes.