Life begins at 40 . . . sometimes
I was approaching forty when I went for aviation. I started flying when I was 31 twenty years ago because I always wanted to fly. Six years later, there started to be news about an impending pilot shortage. I already had most of my ratings. I asked aviation people I trusted if I was too old to start a professional aviation career. They said no. Two people I knew who were just ordinary guys like me had gotten on with commuters. I figured that if they could do it, I could do it. All I ever wanted was a commuter airline job.
I finished my ratings, applied to commuters and freight, and got nowhere. I had 1000 hours and 50 of multi, but this was during the mid-to-late-'80s hiring boom. I finally got a job at ERAU as instructor. All the while, I was applying to commuters. I did so primarily because I saw that commuters were picking up Riddle instructors whose only experience was flight instruction. I received few responses, and those I received were polite little rejection postcards. I built up enough time to meet the commuter mins of the day, which were 1500 total and 500 of multi - less than the commuters require today - and still received few responses compared to my paper output. I did have five commuter interviews, but was never hired. A recession and the Gulf War hit, which essentially stopped hiring. Before then, people nearly half my age and with fewer quals were getting the commuter jobs. I blame age discrimination in large part for my not getting a commuter airline job. I also discovered that there is no pilot shortage, nor has there ever been a pilot shortage.
I write all this in hopes of giving you some perspective. The long and short of it is, well, you got it right, don't expect a job with the majors. You might hear about folks past forty getting on with the majors. The truth is these are extremely experienced and qualified pilots who started young and have flown for years for military, corporate, freight, frax, and regionals. You need regional experience to get Part 121 time to be competitive for the majors. You get that at the regionals and, based on my experience, I feel that regional airlines practice age discrimination. Their conehead recruiters evaluate by the numbers, and older persons who are obvious career changers don't fit the profile. Once again, I base my opinions on strictly on my experience.
Having said all this, there is still plenty that you can do in aviation. Nothing wrong with being a career instructor. Aviation needs dedicated career instructors. My Chief Instructor at Riddle was a career Navy man before he went for aviation. You can fly Part 135, freight if you like it, tours, fractionals. Set reasonable goals.
You can finish your ratings any number of ways. Most people go for their Instrument after their Private. Then Commercial. A very good way to do it is to knock off your commercial cross-country requirements in a single. Then go for Private multi. Then Instrument and Commercial multi. Getting your Private multi relatively early in your training lets you log multi PIC, which is extremely important. After finishing your Commercial-Multi-Instrument, you can add on your Commercial single in a fixed-gear airplane, such as a 172. Then you can get your CFI ratings. Some people take their initial certification in the multi right off their Commercial-Multi-Instrument. They get MEI and CFI-I. I like that idea, because you're fresh off your commercial rides and the knowledge is still fresh, and it makes your expensive multi time cost-effective.
Hope these thoughts help. Good luck with your plans.