Welcome to the forum.
Being in your position, having not done any aviation maintenance experience previously, there's only one path to gaining a Part 66 license and that's through doing an apprenticeship to gain a trade certificate then complete the diploma for a license without type (if you only want to work on GA aircraft, a type rating "might" not be necessary).
Aviation being a highly specialised industry, you may get some credit for what you've completed already, however I've seen guys who'd done a full RMIT Bachelor of Aeronautical Engineering have to do the full apprenticeship as the emphasis is on the handskills over the theoretical knowledge.
To do the modules would require you to either complete a full time apprenticeship with an AMO or to study the modules yourself at your own expense. Seeing as you already hold a higher qualification than a Cert IV, this would mean paying full fee which, depending on the organisation, can be up to $15,000 a year for full time study. I can't say what the costs to study online are, but I'd assume they'd be in the vicinity of $1000 per subject. To complete a mechanical Cert IV is about 24 subjects, avionics is 30 subjects (the B1-10 modules are often broken down into sub-modules, so it's not just 12 or 13 subjects .
If you complete the modules on your own, you'll still need to complete a journal of industry experience and have it assessed by an approved maintenance training organisation. This would require you to work in an AMO and fill out the journal in the range of tasks required for your trade and have all entries certified by the certifying LAME. Once all modules are complete and your journal is completed and assessed, you'll be issued with a Cert IV in Aeroskills in your chosen trade. That makes you an AME and typically takes around 3-4 years of full time contact, either work or study.
A few extra subjects and a few extra competencies in your journal will net you a Part 66 license, either B1 or B2 (there's no such thing as a B3 license). B1 is broken into four subcategories. With what you're talking about, you'd be looking at going for a B1.3 license (if going mechanical path), which is fixed wing, piston powered aircraft. If you've just completed an apprenticeship of 4 years, that will equal the experience requirements for a Cat B1.3 license easily. However, the entries need to demonstrate that you've been working regularly in the industry for 3-4 years, not just a 'one day a week' kind of thing.
Once you hold one license, you can RPL that knowledge to greatly reduce the workload to gain another category. However, once again, it's the practical experience that counts. The only way to log enough time to gain both close together would be to run parallel journals, one mechanical the other avionics, as the tasks are very different and hence the competencies are different codes, so you can't log B1 experience by doing B2 tasks and vice versa. Having filled out a full avionics journal and recently been granted my B2, I can say that that logging parallel journals sounds like insanity to me!
The licensing system is also such that there are currency requirements, meaning you are required to exercise your certifying privileges a minimum number of times over a certain period of time to retain your certifying privileges. Ultimately, the system is designed so that, to retain certifying privileges, one has to be working in the industry.
My suggestion would be to start out self studying the modules online through such companies as Aviation Australia or perhaps ATAE and see if you enjoy it. If you do, you may consider making the jump into the industry with a full-time apprenticeship. Without the logged experience, gaining a Cert IV, let alone a license, is impossible.
You mentioned that you've done a fair bit of reading, so I'm assuming that includes the Part 66 Manual of Standards. If you haven't read it yet, read that first.
Any further questions, ask away!
Cheers,
John