Is ATPL a bridge too far?
May I suggest .. no ... and yes.
No ? - the technical material is not intrinsically difficult so one certainly can get a suitable range of texts and study up in isolation to get a good handle on the subject material. The only real difficulty in doing this is the self discipline involved in making it happen. Sitting in on a real class, run by a good instructor and supplemented by home study, however, is always going to be the easiest way to get through study requirements.
Yes ? - historically, the exam questions have represented a bit of a sub-culture. I well recall my first attempt at ATPL (then referred to as SCPL) Performance (which is one of my engineering disciplines) back in the very early 70s ... I studied for this totally in isolation and knew the technical work inside out ... rolled up for the exam and I might have scored a mark for getting my name right but the questions appeared to have absolutely naught to do with the various standard texts (cited in the then ANOs) I had swotted. Hmmm .. a bit of a problem ?
Some asking around and I discovered that one could purchase worked old exam question papers. I bought a few of those, got an idea of what the examiner thought the subject was all about and promptly passed the next sitting of the exam with no difficulty. My approach for the remaining subjects was the same .. self study in the books, a few past papers to adjust the bookwork for the examiner's approach .. and the subjects went by in short order.
So, what it all comes down to is this - there are two aspects to getting through these exams - first, you do really need to learn what the subject is about. Having done that, second, you need to get a handle on what the exam questions are about. Having done both with a reasonable degree of application, the exam pass shouldn't be beyond the average student's grasp.
Have a look around at some of the folk who have passed the exams. Some are very bright and had little trouble, others are not the sharpest knife in the drawer but, with a fair application of effort, have done the work and got the pass. In later years I spent a lot of time lecturing in CPL/ATPL subjects and my favourite students, just about always, were those who had some difficulty but kept their noses to the grindstone until they overcame all and sundry difficulties to get that elusive pass.
Which is the better way ? Your pick. Doing it by self study and then some exam question review is hard work, generally takes a bit longer, but may save a few dollars. Doing it by sitting a course (class is a lot easier than on-line or correspondence) should take less time, should be a bit easier, but involves a few more dollars investment.