It is a great trip that I have made a few times. The FBO's at St. George have come and gone so I do not know who is there now. Parking on the ramp is ok, but it is not unusual to have sand and wind storms and the airport has little protection from it out in the open desert. Last time I was there was two years ago and they had just cleaned up from a big blow. There was sand piled two feet high in places. I secured a cheap hangar from the FBO but they are now gone.
You can do all of the Grand Canyon corridors in an easy morning and Zion too. Lot's of great things to see. Get the VFR chart. Easy. Morning is best for smooth air and early morning gets you the nice shadows.
My normal flight plan has a fuel stop and leg stretch at Elko (+- 2.5 hours) then another 1.5 or so to St. George. You can get Flight Following for the Elko leg but unless you are on O2 and above 14k, they will not be able to keep you from there to St. George. I like the Elko route as it is pretty much a straight shot from PDX area and avoids most of the MOA's and for the remote desert, a pretty sane way to go. There are good alternates at Battle Mountain, Winnemuca, etc... Then from Elko you can follow the valley past Ely and down to Mt. Wilson. You can usually pick up Nellis by then, or just bust the MOA (not my favorite thing to do), or make a short jaunt around the MOA which adds little to the flight.
I have gone direct from Burns as well which is very doable. Just a longer leg. You might be able to make it direct in an 8 if you get favorable winds (most of time you will have them going, but not returning). It is very border line in my six and four hours is just too long for me to sit.