Yes, you can leave the CPU, heatsink, ram, M.2 drives in the motherboard as a unit.
Either get an ESD wrist strap, or make sure the first thing that comes in contact with the chassis is your body, not parts.
Example: Removing a GPU. Touch the case, unscrew the expansion slot screws, touch the case again. Handle the GPU while keeping in contact with the case. One easy way is elbows on the chassis while you depress the PCIe release lever. Take the card out. Touch the surface of where you are placing it down before letting the GPU contact the surface. All of that is make sure any static discharge goes through you rather than taking a path through sensitive components. ESD strap does the same thing, but constantly.
Same with the motherboard or other PCBs.
Beyond that, make sure required or not required standoffs are in the proper locations for the board. Don't overtighten things. If you are using a full size screwdriver, stick to using the tips of your fingers.
Don't fully tighten a single screw (unless it is the only screw) Start each screw, then partially tighten opposing corners, then adjacent, back to opposing until you reach a uniform reasonable tightness.
For example, motherboard mounting screws. 6 screws in that motherboard. Put one in partially to orient the board. Add the other screws loosely. Loosen the first screw. Square the board up to rear I/O, then proceed to tighten each screw partially, then return in the same pattern to fully tight.
Yes, the EPS connector. Good advice, if possible plug that in before installing the board. May make things a lot easier with the heatsink already installed. Since your PSU is modular, you should be able to do that prior to installing the PSU.
Speaking of modular cables, do not confuse PCIe 8-pin and 8-pin EPS connectors. Make sure the appropriate cable ends go to the PSU. Do not force a cable if it doesn't want to easily go in, double check first. (Sometimes they can be a bit stiff when new)