30 years ago, I passed the exam for a 100-ton motor/sail license after buying a two-book set from these folks:
I recall the books were 40 bucks back then, no doubt more now? I felt well-prepared and as long as you are capable with paperwork and long applications and physical exams and drug tests on your own, I don't see why it shouldn't work for a 6-pack license.
Only thing is, nowadays the school can give the exam (not so back then?). I had to go to the Coast Guard Regional Exam Center (the REC), which fortunately was right here in New Orleans.
Advantages are you can carry passengers for hire on uninspected ("uncertificated") vessels. And you might snag a job at some charter outfit or sailing school?? Mine is good for Inspected Vessels, so I've occasionally been mate or second captain on a 100' offshore liveaboard dive boat which is useful, fun, and may be a retirement gig, who knows? The license started out as Ocean Operator but is now Master, Near Coastal.
So sit for the "biggest" license your time will justify, go for Inspected Vessels if you can, you're going to the trouble of logs, physicals, and exams anyway, you may end up with a 25 or 50-Ton license (but you need 2 years' time, and more for the auxiliary sail part too).