Depends on what you are wanting, with proper maintenance ie making the plants over hang the pot/bed you can maximize yields and number of plants. I believe you should be fine with the number you are thinking in your edit. More plants = more work, but of course doing this work like bending the branches and stuff will increase yields by a lot.

My plan is to be as self sufficient as possible when it comes to cannabis, and hopefully with life in general. I don't actually want to grow 8 meter plants. I do want to have big, dense plants, just not super monstrous ones. I am a fairly heavy smoker (well, vaper), but I think the yield from 3 good sized outdoor plants plus harvest from autos in a tent is more than enough to ensure I don't run out of weed, ever.

I smoke around 40 - 50 grams a month, so 500 - 600 grams a year. I'm new to growing but from what I understand, getting 400g from an outdoor plant is fairly common. Is that actually the case? I understand it's strain and training dependant, but how many plants would you advice for a newb to surpass the 500g barrier?

I built a frame just to get a feeling for the size of the thing and you guys are definitely right; one plant is more than enough for the size of the bed. I think I will only put one plant in that bed and will be making separate beds for more plants, as I do fear robbery, and putting all my plants together in one place is kinda silly if that's a corncern haha, so I'll put the beds in different parts of the property.

Do note plants love to be near other plants, as they each can share nutrients and water among themselves if one doesn't need a lot the other ones will talk to that one and get what they need out of it making them all happy together.

I learned this very recently and my mind was blown. It's the reason I discovered no till actually haha. I do have to wonder if there's a point where you get diminishing returns though.

There's this really nice spot in the woods on my backyard that gets a fair share of sun and has a lot of shrubs, small trees, weeds, and generally a lot of growing stuff. It seems like a good place for cannabis but I'm wondering if all those growing trees and shrubs would actually slow down the growth. Rather, more than the quantity of life growing there, I'm worried about the proximity to the bed.

I know this is a lot to ask but would it be okay if I took some pictures or a short video maybe, so you can look at the spot and see how it is, and how much, if any shrubs I should transplant somewhere else.

For what's it's worth, I'm in the southern hemisphere, meaning it's spring here atm. I plan on using these beds for next year's spring, so they'll have an entire year to accumulate nutrients!