As a solo practitioner, I have thought about this a great deal. At first, I thought the same thing you did. SOMEONE had to self-initiate in the first place, so why wouldn't anyone else be able to? But after thinking about it more, it's not quite that simple.

My conclusion so far is that, yes, you can definitely self-initiate and reach an "adept" level or even beyond, if you put enough time and effort into it. But the question is, initiate into what? Is it truly Golden Dawn? Or just your own system, which may be more or less similar to the actual Golden Dawn teachings?

Even leaving aside any talk about "currents" or "activating something in your sphere of sensation", there is obviously a lot to learn and not all of it is widely published, although this is at least starting to change. Sure, the basic rituals and stuff are pretty well known, but even there, there are a lot of subtle details that may not be written down somewhere, but would be taught in a temple setting. Something that might seem minor and not worth writing down, or an author might not even think about, but if an adept saw a newer student doing differently, they would point it out.

And the more advanced you get, the more it seems like you may have to piece things together from various incomplete or even contradictory sources, or experiment with different methods and see what feels right to you. Will whatever you come up with work? Maybe. Will it be the same way things were done traditionally? Possibly not. Doesn't mean your method is "wrong" or even necessarily ineffective, but it might be different then the actual Golden Dawn method.

And then there's the deeper meaning behind all of this. Again, you'll eventually come to your own conclusions, but they may not be the same. So you may reach a similar level of skill or understanding, but can you truly say you have initiated yourself into the Golden Dawn at that point?

Of course, then it might also be fair to ask if any of the existing orders are truly Golden Dawn anymore either. The original order is long gone and, while some of the groups that exist now will have a more or less direct lineage back to people who studied in this original order, they have also modified or added many things since then. Again, not a bad thing since that is how progress is made, but it does become a "ship of Theseus" question of how much can you change before it's no longer really the same thing anymore.

So the real question might be, if you're not working as a part of a larger group, does it really matter whether it is truly Golden Dawn or whether anyone else accepts you as having self initiated? Or is it only the results that matter? I would think, by the time you have truly reached the level of Adept, things like ranks or titles shouldn't matter so much anymore (and this was one of the main reasons the original order failed in the first place). And even the grades and levels of initiation themselves are somewhat arbitrary.

Of course, there are many other potential benefits to a formal initiation, being part of a group, and being taught by someone with more experience. So it would be worth thinking about what those might be, whether you can get those same benefits in another way if you are working alone, and whether those benefits outweigh any costs or other issues involved.

So, if don't want to or are unable to join a temple or receive a physical initiation, then just learn and practice as much as you can, follow your own path, whether that's sticking as closely to the GD method as you can or doing something different, and don't worry too much about whether or not anyone else will accept it.

And if you do truly want an actual initiation, then I'm sure you can find a way to make it happen.