George Ingles

Pent

Lest

Neat

I have Fennel planted in some of my orchard areas, but not in my vegetable garden.
I haven't noticed it preventing the growth of other plants in the orchard, except for taking up a lot of room and crowding out other plants when it gets big.  I have only just now read about its supposed allelopathic affect on certain types of vegetables.  The reason I don't have it in my garden is because it makes lots of seeds and volunteer plants come up readily making it weedy in my climate.
 

Growing & processing most grains does seem very labor intensive.  Many crops that can fill a similar nutritional niche are easier to grow and harvest and process.  However, as William Bronson mentioned, the long-term storage potential of grain is attractive.

Disasters like Floods, Fires, Volcano, etc. have the potential to wipe out perennial tree crops or make crop-growing difficult for an extended time.  Having a surplus of grains in storage that last for several years may have kept human settlements alive during multi-year bad weather episodes in the past.  On the other hand, during extreme times, stored grains are more vulnerable to desperate thieves than in-ground potatoes/roots or tree crops.

I have not attempted growing grains on a large scale.  Maize/Corn seems to be the easiest grain crop for harvesting and processing.  
My aim is to fill the grain niche via Chestnuts, Corn, and Potatoes, with some Amaranth and Oats maybe too, and other tree crops.

Also of interest is the work of Wes Jackson and others in developing a system for growing perennial grains in diverse-species Prairie conditions.

Cheers!  Great goals there!
I know next to nothing about growing conditions in any part of Australia.
The climate here is a mix of Cool Wet Winters and Dry Summers - with plenty of Deer pressure and also with increasing risk of wildfires.  No Kangaroos to speak of though we once kept Emus, briefly.

I have attempted a few years growing my own landrace of C. Maxima varieties, but I find Squash Bugs to prefer them dramatically... making things more difficult there - not sure if that is an issue where you live, but something to consider.
They do however often have harder shells that make them more resistant to larger pests, in my experience.

Per your desired qualities of thin skin with dry flesh, I would say many of the C. Moschata varieties fit that bill -such as Butternut.
That sort is my next project for making a landrace, as they seem most resistant to Squash Bugs.

Many kinds of C. Pepo I have grown are often on the waterier side, though not all of them are.
Also, C. Pepo is probably the most-planted sort as it includes most of the Summer squash types, Spaghetti squash, and Jack-o-lanterns - hence any near neighbors' gardens will likely cross-pollinate with them unless you bag them and hand-pollinate.

What sorts of Winter Squash/Pumpkins are most grown in your area?  A mix of those varieties might be a good start.

I am just a beginner at trying to make a landrace, but I have grown various Squash varieties for decades.
A fine goal you have, and good luck to you!

I agree with the Mullein answer!
Wonderful wonderful Mullein!
It looks like it may be on its second year, so if you leave it it will grow its very tall flower spike this season.
The bees and other pollinators adore it.
The flowers can be collected for potent medicine, especially for ear-infections.
The leaves are also good medicine, for the lungs.  
The root is supposed to have pain-relieving properties, though I haven't tried that.
It will make a jillion seeds likely, so it will spread in that area if you leave it there.

“Fear is a strange soil. It grows obedience like corn, which grow in straight lines to make weeding easier. But sometimes it grows the potatoes of defiance, which flourish underground.”
― Terry Pratchett, Small Gods

If you are just trying to eliminate these roots - not harvest, and the tool needs to be narrow and durable, I have a suggestion.
My Grandpa made a tool decades ago to be used as a kind of Spud (for peeling logs).
He basically welded an axe head onto a solid pry bar.  The thing is rather too heavy for easy use in peeling logs, so it sat for years.

Recently I have been employing it to sever big roots of brambles, dock, milk thistle, tree roots, and other such things.
Its weight coupled with the sharp edge makes it pretty useful for certain chores.
It doesn't get the whole root mass out as well as a sturdy digging fork or broadfork however.

It is tiring to use for my gangly frame, but it sounds like that would not be an issue for you.
I haven't seen tools of its kind for sale, but it wouldn't be difficult to make if you know a decent welder.

I think that substitutes for commercial fertilizer are not-too-difficult to manage on a home garden scale, small farms, and even sensibly managed larger farms... many wonderful and smart options are discussed above.

I do think that the commercial mega farms are quite dependent on their chemical inputs however, and they would find it very difficult to transition to alternatives with any haste.  Many such big operations have reduced the soil on the land they control to inert dead dirt through their practices.  They must import large amounts of everything that the plants need in order to get any kind of decent yield.  I believe it would take a few years, at least, for these misused lands to recover the soil-life needed to be healthy again.

I think we need hundreds of millions of small farmers/gardeners taking food-growing into their own hands, using local methods appropriate to their particular situations.  I think that at smaller scale, with many more people involved than currently, the task of providing enough fertility to grow enough food will be much easier.  This too will take time, for new gardeners to develop the skills.
Thankfully, we have access to resources such as Permies.com to learn how now!