In short: it is up to the driver to determine what is doable for them and their vehicle
...and here is my extended opinion about the OB with a careful and alert driver.
Not a car guy, at least not until recently lol, if I were to do this all over again, I would get a 4Runner trail (preferably TRD) and call it a day. But when I got the OB, I was eying only moderate trails like the White Rim Road and the Alpine Loop whereas now I am looking at harder trails. The other difference is that I intended to go unpaved rarely whereas I ended up doing that more and more often...
As is, I technically completed 91 trails in stock/near stock (0.5" spacers, 225 65 AT tires, skid plates, upgraded brakes) Subarus but had enough and just installed a 2" lift last week.
I find Central AZ to be as bad as it gets for a Subaru. I have had far easier time in Southern Utah and SW CO than here. The reason is that in Central AZ trails tend to bifurcate into easy and harsh (not necessarily hardcore with insane obstacles but relentlessly harsh due to the particular mixture of coarse sand and sharp rock) with very little in the middle. The angles and even the underbody clearance seem to be never enough whereas I have never had undercarriage clearance issues on easy and moderate trails in UT or CO (just the occasional front bumper issue).
So...if I were to start all over again with your car, I would get a used copy of the old but comprehensive Peter Massey, Backcountry Adventures Arizona
(the big single volume). Then, I would look for trails rated 1-2 (dirt roads, 1s being easy for pax cars when dry) as well as 3 (easy trails). I would avoid the rival Wells book because it is really Wrangler and truck-oriented. Of course, all ratings can change from year to year as trails change all the time and all ratings are meant for traditional 4x4 vehicles many of which may not have better traction than ours but definitely have better angles, especially up front.
The Big Eye mine, for example, is on a 3-rated trail (Big Eye Wash trail, #5 of the Central Region). I have not had any issues on 3-rated trails and thus the above comment that it offers little challenge matches the typical description of a 3 trail. These are the easiest actual trails.
In addition, low-rated roads and trails (rated 1-3) tend to be regularly maintained and thus while their difficulty varies with weather and season, it is generally constant year-to-year as the grader gets the job done with some regularity.
Once we get to 4-rated trails, things begin to change. First, they are not necessarily maintained. Last fall, I drove a "4" east of I-17 that had definitely deteriorated to a 5, with one spot possibly a 6. That was the last straw and I ordered the 2" lift kit. I took more hits on that trail than on the other 90 combined. By contrast, the famous White Rim Road, discussed in another thread, which is also 4 rated, is maintained at least once a year--and it shows. Second, some 4-rated trails will require a stock car to build a small ledge or fill in a small gap due to the atrocious approach angle.
I would definitely not take a stock car on a 5-rated trail. I have done a few and except when the trail had been improved since the book's publication, I always ran into issues with clearance, especially approach angle but also undercarriage clearance. Even with the 2" lift I am not planning on dealing with 6-rated trails regularly and though I would not by default shy away from a 7 now, that better be a truly special trail driven in company (I am almost always solo). Trails above 7 are "hardcore" and thus beyond the scope of Massey's book or the capabilities of even a modified Subaru (except when extremely modified with custom bumpers, huge lifts, low range for MTs and so on).
Last but not least, I think that the Apache Trail, Four Peaks, Bloody Basin, the Senator Highway, Castle Hot Springs, and Agua Caliente pretty much cover the landscape around Phoenix with a taste of offroad and a considerable remoteness factor without the need to engage in rock crawling etc.
The stock tires are adequate for 1-3 rated trails but even so the handling is much inferior to that provided by AT tires. I am not sure I would take a car on street tires on any moderate trail in Central AZ. Too many rocks--and sharp ones to boot, this is not tire-friendly slickrock.
NOTE: though I personally consider returning on I-17 on a Sunday night more dangerous than any offroad I have done--and I have driven lots of shelf roads etc--you need to remember that one misjudgement even on an easy trail can lead to death or severe injuries due to tight switchbacks, shelf roads, etc. Not all tight spots are obvious and I have found the danger greatest at otherwise easy trails (because it is easy to get distracted). Also, never delude yourself that a Subaru can handle clay mud any better than a 4x4. Neither type of vehicle can; never drive in wet clay mud, the car becomes a sled.
Also, though I am almost always solo, it is highly advised to travel in a group. Carry at least a foldable shovel, be able to replace a tire (means also carrying a plank to put the jack on since surface may be uneven) and bear in mind that Godzilla tape can temporarily "fix" dislodged bumpers if it is impossible to clip them back in place on trail...Personally, I have not had a puncture yet but I do not air down P-metric tires unless it is absolutely necessary. If you do travel in a group, trail etiquette requires you to carry an appropriate recovery strap, a tow strap, and two rated shackles (there is an eye bolt where the spare tire is and a second one can be purchased from a dealer for about 20$).
You will want lots and lots of spare water, extra food, and very comfortable hiking shoes. I finally got a DeLorme InReach since I thought I had been pushing my luck enough, especially with kids on board. Basically, once one gets beyond a 3-trail, things get a lot more complicated (I also have traction mats, offroad air jack, and even a pair or PSPs for bigger adventures).