There's always a chance for a low-level encounter to turn lethal - that's DnD.
That kobold could hit with a crit, deal enough damage for a massive damage death, and the PC fail it... one dead PC of 12th level - from a 1st level kobold.
But it sounds as though you want to change that. That's fine (green lights as mentioned above). The world is fantasy. Realism doesn't enter the picture. Do you calculate the tidal effects + axial tilt + mean altitude above sea level to determine the probability of the season and day-to-day weather? Have you computed the effects of overall climate on the terrain, or do you have forests in the lee of coastal mountain ranges where you should have deserts?
You've got the right ideas, without having to worry about realism. What you're actually concerned with (from what I've read in your posts) is story flow... you want appropriate challenges for higher level characters without any real stretching of the 'willingness to believe' on the part of the players.
The best way to do that is over-arching story lines. I long ago moved from the traditional plot formats to a tree like flow chart outline of multiple plots interwoven. I typically plan out 2-4 major arches, with about 20 to 40 minor plots/sidetracks going on underneath them. I plan these out in chronological order of events, and tack times and locations to the events. Essentially, I plan as to what the opposition does if there are no heroes - all the way to the end. Then I launch my players into my world - which is exactly the same type of world, not anything like the DMG world, but one where dangers abound and are not artificially tied to your level. Whenever the actions of the heroes creates a new branch in my plotted outline, I take time to re-invent new actions/reactions of events a ways down every branch I can conceive them creating at that point. if I'm lucky, I can get them talking about the plot... not only does this give me clues as to what they are likely to do, it fuels my creativity and spurs me into more complex plots
As for magic items, you can take the advice above - it's good, as far as it goes. My worlds typically take this to the next level beyond that. Magic abounds, and even a first level character can find a powerful magic item. But all magic radiates it's presence, and the more powerful the item, the stronger the presence. Likewise, the stronger the magic user, the more the user notices these auras. So a first level party with an artifact will often find themselves targeted by the very powerful, some good and some bad.
I think you've got the right ideas about your world. It'll just take practice to get it down right. Those over-arching storylines are the key. I often have about 5 or 6 seperate layers of plots, of varying importance and unconnected, and each has multiple 'events' that can trigger the plot, or change it drastically - like the brigands that take over a village and are kicked out by the high level party as described above. Those brigands were actually nobles sons - deal with the kings wrath for hurting his children, only to discover his wife is an agent for good who'd manipulated the noble children into freeing the village from tyrannical mayor, and is now most wroth with the party for disrupting her plans. To redeem themselves, they are sent on a quest that's even more wrinkled with plot twists
Plot lines are your saving grace for keeping even high level characters interested. You said it yourself, when you mentioned your cavalier riding back through those dangerous woods... you mentioned 'you had to', and then described your feelings. You had to due to plot lines, and the feelings were derived from the setting
You know what you're doing - you just need to realize that and practice it
