Pursuing a PhD is such a personal question, it’s unlikely that strangers on the internet can provide you with the answers you seek. That said, as someone who relatively recently earned a PhD in a somewhat similar field at a U.S. institution, I’d offer some strong cautions.

1. A PhD in history or a humanities field is largely professional training in how to be an academic. If your goal is to be a popular writer or do research in another setting, then it’s probably counterproductive to do the PhD, and it may even teach you to write in ways that make your writing less accessible.
2. As you note, a PhD in history will certainly not advantage you in most jobs. A PhD is a minor positive in the nonprofit sector, but fairly harmful in most other endeavors. This is true no matter where it’s from. You have to explain it away in some job interviews. Ironically, a PhD from Harvard or Yale can hurt you even more in some jobs because it’s seen as pretentious.
3. Pursuing a PhD takes you out of the job market for somewhere between 6-10 years. While you can earn somewhere around $20,000 to $30,000 as a graduate stipend, it’s not the same as a full-time job. Unless you are independently wealthy, pursuing a PhD without the intention of going into academia is likely to be a financially precarious choice.
4. Even before the pandemic, the academic job market was, as you say, awful. The dream of being a tenured professor was about as realistic as a garage band making it big. Right now, it’s probable that many institutions will have to make lasting cuts to deal with the pandemic, and it will likely get even worse. U.S. political and intellectual history is a particularly tough subfield with regards to employment too.
5. If you truly love a field of study, nothing is stopping you from reading or knowing about it outside the strictures of academia. You do not need to pursue a PhD to legitimate your interest. A PhD is mostly useful if you feel you have to publish your own material in a field. For at least some people, pursuing a PhD does kill their love of history by making it too much like a job.
6. When seeking advice, I’d be cautious about hearing from your undergraduate professors, who may have heard how tough academia has gotten but often have a “survivor's bias,” as they all got jobs. You might want to read The Chronicle of Higher Education for a more realistic view, or read some of the recent wave of academic “Quit Lit” (historian Erin Bartram’s blog post is one example that has gotten attention).

If you opt to pursue a PhD, do it with eyes open. Every competitive PhD program is made up of people who were shining stars in undergrad. Don’t take your interest or success in a field as an undergraduate to be determinative of what doctoral work will be like. I usually don't regret having done a PhD, but I wish I had been more willing to consider the considerable downsides.

As for your question about sources, I have three suggestions on U.S. intellectual history. If you don’t already have a copy, track down the essay collection The Worlds of American Intellectual History, which has essays by many of the leading figures in the field. You also might benefit from listening to the podcast Trotsky & the Wild Orchids (named after the Richard Rorty essay). It's by two intellectual historians, Ray Haberski and Andrew Hartman, and often talks about trends in the field. Finally, you’d want to stay up to date with the Society for United States Intellectual History (S-USIH, pronounced Sushi) and attend its conferences when you can. Your interest in liberalism would be right at home in S-USIH.

I, too, wanted to pursue a PhD in history. At the advice my professors, I didn’t. The only reason for a PhD in history is to teach at University level or to publish research.

I ended up switching to a law degree; history majors are looked on quite favorably at many law schools (according to my law advisor, history professors and academic mentor).