If you can get out of the metro area, you'd probably get better results in parks and schoolyards. Phoenix had a 1876 park, Chipotle said here or nowhere. Saw a early Phoenix map about WWI destroying park and making new one half a mile away. This is common in most rapidly growing cities in the south.
Some of the suburbs are 100+ years old but many parks means many park & rec depts have huge budgets so renovations are frequent. If you want to know the oldest suburbs or oldest towns outside of metro, I could probably help.
I used to own many Ace 250 units, none would give an accurate ID beyond 4" on dime.
Shortly after I bought Garrett AT Pro, I got dime 7" deep with perfect ID on every pass.
Also, my ground is mostly just moderately mineralized, so an Ace 250 might not even get 4" on a dime if you have high ground mineralization. An AT Pro might cost you $350 used. There are better detectors than the AT Pro, if you can spend up to $500 used or $649 new. But as long as you can be satisfied with the Ace 250, keep trying!
Another detector to consider is the Bounty Hunter Land Ranger PRO, I've seen then as low as $140 used on detecting forums. Also, tekneticst2.com has most models on sale. BH LR Pro there is Teknetics Liberator for $219, 5 year warranty to original owner. It's the lowest priced model with manual ground balance, so goes deeper in bad ground. It also has high disc programs with no ground balance required.
Teknetics Eurotek Pro as Tek Minuteman, $199 5 year warranty to orig buyer with 8" concentric (round coil). It has display that blinks iron, so you get fooled less. Very few controls, so not much thinking required.
The Teknetics Digitek last I looked was $129, has variable volume, variable discrimination, variable sensitivity, notches, and non-motion pinpoint. You must specify
20" lower rod (default) or request 26.5" lower rod if 5'5" tall +). Maximum depth on coins is about 7.5" on dime or 8.5" on quarter. 5 year warranty to orig. buyer. Made in El Paso, TX by First Texas, that also makes Bounty Hunter and Fisher.
Just for when you tire of Ace 250, don't mean I order you to buy now. Best wishes.