the issue is not with cutting wood, but rather then cutting pressure-treated wood.. in the past HD have refused to cut it (I am in NJ) - there are some chemicals or whatever that can be released when you start cutting pressure treated material. So call your local store, repeat several times that you are referring to pressure treated wood, and let them confirm for a fact if they will do it before you make the trip..
I have used 2x12's for decking in many of my piers over ware and outdoor decks over the last 40+ years in South Louisiana. I use 2x10's on 2 foot center's and they work exceptionally well for normal foot and light equipment traffic. I have to repair and or replace about every 20 years or so. Screw and Glue down the ends to delay splitting and cupping. And double plate [2x4 attached to one side of 2x10] the supports that carry the split between the board ends so each side gets a full 2 inch surface to be glued and screwed attached. Angle cuts require more cutting, labor, and maintenance.
Pressure treated is not light, but that being said, some are heavier than others. Depending on length, have someone help you load and unload. HD always offers that service at the store.
Yes, if you have circular saw, it’s a very easy task. I first measured the length desired, marked with a carpenters pencil and used a speed square to draw a straight line.
MAKE SURE IT IS PRESSURE TREATED AND FOR GROUND CONTACT. I USED 10 FEET ON MY GARDEN.