Getting a bit lost with what to do; which green manure should I (or shouldn't I) sow, and where ?

Hello, as the spring is getting closer and closer, I am readying most of the garden for this season.

I have a good spot, hugelkultur like, which is about 2.5 sq meter. I recently made the hugelkultur, and I intend to plant perennials medicinal and aromatic plants.

I have another spot, bigger, where I had buried some logs (mostly big twigs) about a year ago, where I intend to plant a lot of medicinal herbs which help in one way or another with sleep and dreams.

I have yet another spot, with more woods buried (I think a pattern is starting here), where last year I had sown some physalis and Mexican tarragon.

And lastly, I have a spot, with some wood buried too (from last year too), where I intend to plant watermelons and a few other companions plants, mostly on treilis.

I am getting a bit anxious with everything, and despite it all being a huge experiment, I still find myself thinking too much, and getting stuck. So, to help with this, I want to know what would be the ideal thing to do.

The soil is mostly clay, although I loosened it. I removed most unwanted plants there, and tried to keep the ground covered in some way (but I probably didn't mulch enough). I live in a 8b USDA zone.
I have hemp mulch available, phacelia, melilotus, mustard, white clover, camelina and alfalfa seeds. I plan to add some compost to the watermelon patch, but I want to limit using it for other beds as I'll be planting medicinal plants and am not sure about the content of said compost.

I can get more "good" compost for the medicinal beds if required. All of them, except the new hugel, were cultivated last year.

What should i do ?
So far I plan to sow phacelia on the dream/sleep bed, let it grow enough, then chop it and put compost on top of it, then plant (as most plants there will only be available in May).
For the watermelon, I'm thinknig about adding compost, then perhaps having either some straw mulch or sow a green manure (but which one) ?
And for the other beds, well I have no idea.

What would you do ?

It sounds like you are doing great.  Gardening can become addictive, and we can sometimes over extend ourselves.  I can totally relate.   I also have clay soil. I have found covering the ground with wood chips to be very beneficial.   There are so many plants that are great for chop and drop.  It all depends on what will grow in your area, and what works for you.  A few examples are the ever popular comfy, growing medical herbs, you may already be growing it.  It makes an amazing green manure.  Everyone raves about this being a super easy plan to grow, and it has many uses.  I live in N California zone 9b, and I have had a hard time keeping it alive.  I like borage, I planted seeds a couple of years ago, and have had it ever sense.  Clover is supposed to be a super cover crop, peas are a great for nitrogen. And on and on.  I covered my hugelkultur with what was supposed to be straw.  Not long after, I started getting what I thought was grass, but was actually the seeds from the "straw". I was upset at first, but it made a great chop and drop. You just never know.    Maybe try a bunch of different things, that way you will be able to truly find what works for you.  Good luck, and have fun.

I'd say whatever cover crop that is easily available for you and fits your needs is the ideal one. A friend went nuts to get buckwheat (very hard to obtain here) and it didn't do well after all his efforts. Meanwhile I went to the feed store and bought winter oats and some whole dried peas at the market, both cheap as dirt, and they grow great in the winter.
I'm a big fan of going to the places that sell beans, peas, grains, etc and sprouting what they have for sale (sunflowers, sorghum, peas, beans, millet, rice and oats with hulls, hulled barley....). If it will sprout, it will grow. Maybe soak overnight and then set it out.