You are right about doing too much trimming now. Your task in heat of summer is just to keep everything surviving through it... keep watering, provide some shade as needed. If the place you moved into is overgrown, this is a good time to get it weeded out and cleaned up, put some mulch down to help retain water in the soil. And be planning what you want to do with it all.
Once the heat breaks at end of summer, early fall, you want to trim things back. If you have bulb plants (day lilies, irises, etc) that are crowded, fall is a good time to dig them up, divide and replant. If you are adding things, lots of perennials, shrubs, and trees can be planted in early fall. They get roots established by winter so they are ready to take off in early spring.
If it's overwhelming, work in sections. Pick a bed or an area to work on and do it a little at a time. It is ALWAYS going to be a work in progress; that's the nature of gardens. I have lived where I am 8 yrs and it is definitely still a work in progress... don't stress about it!
Feel free to ask more specific questions as they come up.