I also completed PhD programs in both clinical psychology (child/adolescent focus) and school psychology. In my case I started in clinical and decided to add in the school psych piece late in the game (although still before graduating,) thinking that the school psych aspect might give me a better schedule while having/raising young children, and still allow me to work as a psychologist with children (which I love). That said, I ultimately decided to be an at-home parent after graduating with my PhD in my early 30s and now that I am looking to go back to work in my early 40s, I do not think that I could be happy working in a traditional school setting. Who I am has changed (in no small part to raising 3 children who all have special education needs), and the role of a school psychologist has continued to evolve. In combination, I think that there would be a poor fit and I would be at risk of finding myself in a situation similar to the one that you describe (frustrated and uninspired).
.......
Sorry, I just re-read and edited because it looks like your goal is to work as a therapist with minimal additional schooling/hoops to jump through and you are ambivalent to working with children vs. adults....If therapeutic work is your bliss and independent practice is a goal, maybe a psychologist isn't the right fit and some of the counseling roles suggested above would be a faster path to that end. I think that given your experience in school psychology you might want to "try before you buy" when making this change. Learn more about the counseling options available to you and the training involved in achieving them. Learn about the jobs you are considering and talk to people doing them (are they happy? would you be if you were doing what they do?). I would just ask people in your area if they have roadblocks to a robust practice that you need to watch out for (problems with insurance reimbursement, market saturation like you found before, etc). Can you shadow someone (probably not due to confidentiality issues, but worth asking)? Can you volunteer? In any case, I wish you luck in finding greater happiness in your career.
.....
Editing again because I just re-read everything that you've stated above and I'm going to be so bold as to suggest that despite what you've said, maybe you do not really want to be a therapist (unless you just wrote this when very upset and are simply blowing off steam rather than speaking honestly).
To reflect back some of what you shared above:
-You do not want co-workers or to have anyone dictating how you do your job....You need co-workers (and likely a boss) unless you go into independent practice. Unless you are so talented and your area so affluent that you do not need to accept any insurance, you will have to deal with insurance companies (in your independent practice) who may challenge your desire to avoid dealing with restrictions of any sort.
-You suggest a desire to do marriage and family therapy, but you acknowledge that you do not work well/comfortably with others. Marriage and family therapy is all about improving interpersonal dynamics and it sounds like this isn't a strength of yours.
-You speak of doing independent psych evals for the school districts, but in my experience these are only done in cases where the scope and complexity of a child's problems are so vast that the level of expertise available within the school district is inadequate to answer the educational questions; generally this is done by a neuropsychologist or the like. I think that few school districts would find it cost-effective to outsource to a non-licensed master's level school psychologist rather than use in-house talent (but perhaps your area is different). If you are suggesting that you want to earn a PhD/licensure and then do independent evals for schools as your next career, I would suggest that you reconsider this goal. You state that you don't like working with kids. This would be working with kids. Additionally, this is a big deal for parents/children. Families need to advocate hard to get an independent eval done at the school district's expense (sometimes hiring an advocate or attorney to make it happen). They often wait a long time for one, during which time their child is languishing in school. The person doing the evaluation should be really talented and skilled in working with kids who may be challenging, resistant, and struggle with the testing tasks. If you don't like the kids, they'll figure that out and it will not increase compliance or the validity/utility of your findings. It is a privilege to serve children with special education needs in this way, and should be treated as such. It should not be pursued because that's "where the money is."
-You state that, "I don't like being forced to work alongside others that I dislike." Clients are going to be challenging/unlikable at times. It is often part of the process. You mention juvenile detention centers as a possible job site in terms of this transition. This will not be a likable population by-and-large and also involves kids, which is a population that you state that you do not like. It seems like this option shouldn't be on the table.
Finally, I cannot ignore the language that you use throughout your posts, which is very victim-based. You were "lured" into a field that you don't like, crammed into poor work conditions, left at the mercy of "rinky-dink" school districts, and are "stuck" working with kids. You speak of money on several occasions, but never about a desire to serve clients in any capacity. This is a helping profession. There is no shame in not being oriented toward/fulfilled by helping others, but it is not really ethical to work with clients who you do not have an intrinsic desire or talent to help beyond your personal economic gain (I am not suggesting that this is your situation, but it is the gist of the information that you have provided). Moreover, it sounds like using money as a motivator has landed you in a field that you hate by your reports. If it is likely that you will hate being a different sort therapist, money will no longer be enough to keep you going in that role either..
You reflect upon experiences wherein you realized that school psychology was a poor fit, and it may just be that therapy is a poor fit (kudos to you for recognizing this and taking action to change careers if it is the case). Many people start graduate school in some area of psychology and learn over time that this is not what they wish to do or not a skill set that they possess. That is totally okay, but accepting money for a service job that you do not like to do and may not do well in the available circumstances, is worth reflecting upon. I continue to wish you luck in finding a career path that brings you happiness.