Here's how I set a toilet every time.
I set the toilet in place without the wax seal.
Always install new bolts and secure the bolts to the flange with nuts & washers.
Then put you weight on the bowl and check to see if it rocks.
Some times the flange is to high. You want the bowl to sit firmly on the floor , Not the flange.
Then I place shims until it feels firm.
Old school method was to make copper shims by smashing pieces of copper tubing, or a few copper pennies, but now they have plastic shims that work much better.
Lift up the bowl set you wax ring and reset the bowl.
Other problems such as the flange not secured to the floor and it may flex upward when you tighten the bolts. Shims won't help.