Personally, I have no problem with anyone 'cross-posting' anything lol. Each of the many forums on this site have different participants. Yes, some do cross over to many different forums as interests vary, but lots of input provides options. Sometimes it causes some confusion, but in the end, it will be your choice of what to do in your circumstance.

It does help though to find the most appropriate forum for a question.

That said...

The mix you describe is often referred to as Mel's Mix. It is a common soiless medium used by Sq. Ft. gardeners. It has both up-, and down-sides.

I'm not a fan of using vermiculite outside, but that doesn't mean it's a 'bad' choice. It's the word 'needed' in your comment. No individual ingredient is 'needed'. There are many choices of what might work to accomplish your goal.

I also rarely use peat, but it too has it's place.

My first question to you would be... why do you say, or think that your native soil is 'useless'?

The easiest (and arguably the "best") soil mix to use for a raised bed is just plain old garden soil enriched by a modest amount of compost. If you don't have garden soil available (or not inclined to use it if you do have, for whatever reason), then purchasing top soil and amending that with the compost will work equally as well.

I am not a fan of Mel's Mix for a number of reasons. First, unless you add lime, the peat moss makes the mix unnecessarily acidic. Second, vermiculite is expensive and under most circumstances, does not make a good soil amendment as it readily collapses and compacts once fully saturated. This impacts good drainage. And finally, the mix has NO mineral content, which is what real soil (garden 'dirt') provides. And those minerals are just as necessary for plants to grow and thrive as the other, more familiar plant nutrients like N,P, and K.

You do need some soil there or it's going to drain very fast and get dry in hot weather, and all that organic matter will continue to decompose, and it will shrink.

I avoid peat moss because it's hard to wet, can shed water when it dries out, it's not nutritious (like compost is) and it's imported if you live in the lower 48. Other than that it's fine. :-D

I too am curious about your native soil. Is it sandy, silty or clayey? Have you tried growing in it and failed or was it just too hard to dig a hole in?

In on this. I'm putting together raised beds as well for the first time ever and know that the soil mix is important...I don't want to mess up.

I've seen 1 part bagged compost/1 part topsoil or native soil/1 part coir or peat moss....with added in earthworm castings, rock dust and mycorrhiza. Fertilizer added into each plant hole. Does this sound about right?

I've seen people adding bagged potting soil to their raised beds but I'm tired of the potting soil mixes. They all contain too much bark/too many twigs. And as I understand it, soil in raised beds doesn't need to be as porous as soil in containers/pots...?

For an 8 foot by 3 foot by 20 inch deep raised bed you will need about 81 cubic feet (or about 3 cubic yards) of soil so bagged soil is much too expensive to use. Look for something that approximates loam, 45 percent sand, 25 percent silt, 25 percent clay, and 5 percent organic matter. Some locally owned garden centers or landscape suppliers are custom mixing soil, now.

Agree you don't need potting soil, it does indeed have a different texture.

I'm not sold on worm castings, rock dust and mycorrhiza:

Worm castings are great compost but any kind of compost can do that job as far as I know. If you have or can get them inexpensively, they sure don't hurt. I have not tried to buy a bag because I make my own compost.

Rock dust is what soil is made of, so, if you have soil, you likely have all the trace minerals needed. We had a huge discussion on rock dust here just the other day. If you have done a soil test and find your soil is deficient in a certain mineral - which does happen, just not very often - then you should amend with that mineral. Otherwise you're throwing fruit salad at...fruit salad, and paying for the privilege.

As for fungi: they are extremely important for soil and plant health, but they should already be present in soil and compost. Fungi feed on decaying organic matter, so if you have plenty of compost in your soil, you'll have all the fungi you need. But again, it doesn't actually hurt anything. I'm just not convinced it helps if you have otherwise healthy soil.

Keep in mind that peat moss is a non renewable resource that really does little to improve soils.

Add more compost? Maybe. What is the current amount of organic matter in the soil in that bed? If it is not yet at the 6 to 8 percent range add more organic matter which can be compost or shredded leaves or whatever other kind of organic material is available.

Nicely said, tox :-) This is a point that is frequently overlooked or misinterpreted. Adding quantities of compost or other OM in excess of the desired % of soil OM is a very common and expected practice. Once the OM breaks down, you should be at the appropriate soil OM level. The two concepts are often considered to be the same, which they are not.

"I still have about 3 inches I can put in there, should I add some more compost to even the ratio? "

Make sure you leave room at the top for mulch - I like 4 inches or more. Also, as you mulch and that composts down on it's own, the soil level will rise year by year. That's why even leaving six inches is fine when you are creating your beds.

Leaving 6" - or even 3" - in a raised bed that is only 10" deep to begin with is not allowing much room for root development. And it takes longer than a single growing season for plant roots to penetrate the underlying soil so that may not be available for additional root growth for awhile. Using a mulch in a raised bed during the growing season, particularly one that is used seasonally to raise veggies, is not a requirement. You can address both the weeds and the moisture evaporation a mulch will help to control manually and leave the mulch for when the bed is fallow.

Personally I would fill up the beds with whatever soil mix you choose now and then plan to top off or add at the beginning of next season. There will be some shrinkage depending on the amount of OM added and with the removal of your harvest.

Any gardener will always need to add much more organic material, compost for example, than the desired 6 to 8 percent because the Soil Food Web will digest that material to create the organic matter that you want. It takes about 10 tons of organic material per acre to raise the level of organic matter in soil1 percent.

Soils will need added organic material every year, if they are in a good healthy state because of the activity of the Soil Food Web digesting that material. The residual, what is left of that organic material is the organic matter that is somewhat stable in soil. I have seen many people here complain that you need to add organic matter o soil every year and yes you do. Ma Nature provides for that but too often many people look upon that organic material as waste that needs to be removed when it should be put back into the soil, if not mulch mowed in then at least composted. Those leaves from trees and shrubs and the plants have valuable nutrients along with the benefits that organic matter in soil provides.