For the Job you are planing, your method will be fine.
Some people on here seem be getting confused between the weight of the average shed and contents and a full blown construction project.
Trust me i have done hundreds of shed, summerhouse and workshop bases over the years and never had to go back because of cracking.
If the ground is firm and well drained then it will be ok.
I live in the Fens and round here the ground moves and cracks with shrinkage.
As long as you place the reinforcement in there, i can assure you it will be fine for years.
As for the of anti Frost additives (its already in there if its from one of the more well known firms), Gemix, Cemex, Lafarge.
Most small sheds are usually put on Slabs with a good 3" 20mm type 1 or Limestone which is whacked down, then a 6 to 1 mix of Sharp sand and OP cement, with some feb mix plastisising agent in the mix to increase pliability in the mix and increase adhesion to the slabs.
Haunch up mix at a 45 degree angel all round the edge of the slabs to stengthen and bond the slabs together.
In the relms of groundwork and contruction they use a Gen 0 semi dry Kerbing mix for setting and haunching Kerb stones.
By the time you have paid out for the Sub base, whacker plate hire, Sand, cement , Slabs and hired a cement mixer , the readymix is easier, faster and in the end cheaper!
Before anyone says, but your a Tree Surgeon ! I'm actually A Landscape Gardener, Groundsman and Hard Landscaper and a Blo*dy good Arb Man.
A man of many talents, mostly legal ,Lol