It’s always puzzled me that evolutionary psychologists only seem interested in relating human social behavior to that of other apes, and therefore can only see the alternatives cited of monogamy or polygyny.
Looking more broadly at animal social systems, there are many other taxa that typically form strong pair bonds, with biparental care, complex social networks outside the pair, jealous mate-guarding males, occasional threesomes where the alpha shows varying degrees of tolerance for the beta, and numerous secret affairs by both sexes. It’s called social monogamy, and it’s associated across species with evolution of extraordinarily large brains, creative problem solving, tool use, and language-like behaviors. Many people think this happens because the social monogamy situation creates intense social selection, which becomes a runaway process until countered by natural selection.
The animals that do this are mostly birds, though, and I guess psychologists are not interested in them. It’s hard enough to get acceptance for comparing humans to apes; comparing humans to sparrows isn’t going to win you any friends.
After several years as a post-doc I am facing a similar choice.
If I understand correctly you have no research experience so far. I’d strongly suggest completing a doctorate because:
you can use that time to network and establish a publication record
most advisors will allow you as much freedom as you can handle, particularly if you can obtain a scholarship so you are not sucking their grant money. Choose your advisor carefully.
you may well get financial support that allows you to work full time on your research for at least 4 years with minimal accountability
if you want, you can practice teaching and grant applications to taste how onerous they would really be
once you have a doctorate and some publications, it probably won’t be hard to persuade a professor to offer you an honorary (unpaid) position which gives you an institutional affiliation, library access, and maybe even a desk. Then you can go ahead with freelancing, without most of the disadvantages you cite.
You may also be able to continue as a post-doc with almost the same freedom. I have done this for 5 years. It cannot last forever, though, and the longer you go on, the more people will expect you to devote yourself to grant applications, teaching and management. That is why I’m quitting.