If it's still warm and plants are green, you have enough time to begin the plant removal process. But know that killing with herbicide is usually NOT a one-shot operation. There are many weeds that, while injured above ground, are able to persist below ground and return later. Sometimes, many months later. So getting rid of existing growth is more like a war, not just a single battle. And it depends on what the growth is. Round-up does not kill all plants. Most of those not killed by it will die with the application of a 2,4-d-based herbicide (active ingredient in Weed-B-Gone.) One can prepare them both together at their full strength regular recipes (as opposed to mixing them separately and combining them, which would be half strength) and spray that mixture, which will kill most anything. Even in spite of that, though, there will be some regrowth and a second, minor spray will be needed after it occurs. You might consider spraying now to start the process and then again after the spring flush of growth. That would get rid of almost everything. If you're hellbent on planting something this fall, you can do it, but will need to make sure its foliage is not accidentally sprayed in the spring while you are "getting" the bad plants.

As far as stopping regrowth of the tree you are removing, the process of drilling holes is not necessary. Just cut the tree and paint the perimeter (a 2" wide band ought to do it) of the stump surface with Round-up or 2,4-d CONCENTRATE. Do not paint it with a solution that has been mixed for spraying as this is far too weak. I have had excellent results with this method using either of these chemicals. Use a cheap throwaway brush. And paint it the same day as the tree is cut, not days later. It's possible to have the killed plant pop up new growth from the roots, but cut this and treat with the same process. Round-up (Glyphosate) is sold in many different formulations of varying concentration. I have used this method with 41% or greater active ingredient. It may not work as well with some of the weaker, retail grade concentrates so be sure to check the label to see the strength before you buy.

I am not at all sure you have thought this out correctly. There may very well be any number of valuable plants in your garden than can be easily rehabilitated. And it is extremely difficult (and hugely expensive) to get any sense of establishment or age/maturity when you start over from ground zero. Not to mention the dollars involved in purchasing all new plants for your landscape, regardless of their size and age.

RoundUp is best utilized when plants are in active growth - you are getting pretty late into the season for it to be most efficient. As stated, it will likely take several applications before you get full results. And the larger the plant, the less likely it (the plant) will be efficiently eliminated.

It may be far more to your advantage to hire a professional consultant to come out and evaluate your current landscaping to see what is worth saving, what is salvageable and what should be eliminated. Sometimes just a good pruning or a relocation can suffice. If something needs to be removed, manual removal is going to be far more efficient (and likely less hazardous to any surroundings) than using a herbicide.

And proximity to a residence of a large tree is no guarantee of foundation problems....that's a bit of a garden myth. Most trees have no effect on a structurally sound foundation regardless of how close they are planted. The qualifying aspects would be the soil conditions or the species of tree. I would examine this issue much closer before making any rash judgments about removing the tree. An established mature tree can add considerable value to a landscape, not to mention providing shade relief or heating/cooling moderation. FWIW, injecting RoundUp into a tree is really not the way to go.......for one, it is against label directions and therefore an illegal use. Second, it may very likely have minimal impact, depending on the type, age and size of the tree. If you need to remove a tree, do it the proper way, by cutting it down.