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You can trim it up right at the edge of where you do not want it. Other than that I don't recommend trimming much. You can water after, but they will usually live just fine without supplemental water unless it is very dry.

This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/juniper/growing-creeping-junipers.htm

From the photo and your description, it appears to be a water deficit (drought stress) issue that is causing the die-back. Watering is the only thing that will save it. Water not just at the base of the tree but the entire root zone, which can extend as far as the foliar canopy and more. Don't worry about watering too much for a short term effort to revive the tree. Long term, over the winter and next year, avoid excessive water. But also avoid drought stress; you can deep water once a month in addition to what your irrigation system has been providing.

https://www.gardeningknowhow.com/ornamental/trees/pine/pine-tree-browning.htm

That is completely up to you! If you choose another evergreen conifer type, you probably won't have to amend the soil.

If you choose something that is deciduous, or cannot handle the acidity of the soil then you will want to amend it before planting.

Your local extension service can help you find out what is the condition of your soil: https://www.gardeningknowhow.com/extension-search

Here are some articles that will help you: https://www.gardeningknowhow.com/special/organic/the-importance-of-soil-for-an-organic-garden.htm

https://www.gardeningknowhow.com/garden-how-to/projects/removing-existing-plants.htm

Since your location is within zones 7, 8, AND 9 I will recommend plants for these zones: https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-7/planting-gardens-in-zone-7.htm

https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-8/zone-8-plants.htm

https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-9-10-11/zone-9-perennial-plants.htm

Shubs, including evergreens, drop their leaves is sunlight doesn't reach the leaves. The use of hedge shears to prune contributes to the problem. Tip cuts lead to a profusion of new growth at the shrub's outside edge. This creates a dense, sun-blocking exterior. While hedge shears save time, the shrub must be hand pruned each year. Cut a stem back to where it meets a branch. Prune out enough so that sunlight penetrates the interior. Unfortunately, juniper will not grow new leaves/needles on bare wood. It can still look good from the outside. Use the search feature on this website to find information on how to prune the particular type of juniper you own.

Although I wouldn't know from personal experience, since we don't have them here in the US, but there are several sources that state these will leave Junipers alone.

If they are anything like our deer here, that won't mean much. Our deer will eat anything, even if it is labeled as deer resistant. Your wallaby population may be the same way. Most sources say that there is nothing to worry about with Junipers, though.

This article can help in the meantime: https://www.gardeningknowhow.com/plant-problems/pests/animals/keeping-wallabies-out-of-gardens.htm

Pruning junipers can be tricky because of their "dead zone." If you cut into that, no new leaves will grow. As long as there is a male and female in close proximity, and it does not get disturbed often, you will see a good fruit set.

These articles should help: https://www.gardeningknowhow.com/ornamental/shrubs/juniper/care-of-junipers.htm

https://www.gardeningknowhow.com/ornamental/shrubs/juniper/overgrown-juniper-pruning.htm