seating wall footing depth

hi everybody
need some advice
here in Chicago I'm building new brick patio approx 24'x16' with the wooden pergola in one corner and the 16"-18" high seating wall around it
to make all project sturdier I want to pour the footing for the seating wall. while the pergolas posts will be buried in in concrete below frost line at 48". seating wall footing I will make 6"-7" wide and 24" deep with 2 lines of rebar.
do you think seating wall footing should be deeper? wider? should I put 4 lines of rebar instead of 2? should I put few inches of gravel under the footing and then compact it before pouring the concrete? any additional thoughts you have - please share. thank you for your help!

 

I have 2 smaller seating areas on the back of my deck, built up with dimensional stone.

The seating area, including the paver patio is sitting on 10" of compacted crushed limestone and it's been there for close to 8 years with no issue.

I'm in Michigan so similar area, I'd say your more than sufficient.

 

To prevent frost-heaving of the footings, it's important that what they're poured on is a well-draining and compacted base. And keeping surface water drainage away from the footings also helps minimize frost-heave.