BTEC Electronic Engineering
ICS (international correspondence schools), are a company based in Scotland who
run a used to run a BTEC Electronic Engineering Course (I think it's been cancelled now as I couldn’t find a link to it on their website)? You get sent a pair of folders, containing the course for you to work through, an introductory electronics book, and also you get a lot of support from tutors via email, which is just as well as it cost £800 ($1000!). This was back in the day (2014) before I’d discovered the near-free EdX. Still, the course was pretty in-depth, I think the equivalent level is probably UK college, with occasional steps touching university level; There are a lot of math and calculations to get through, but it's not overly-technical (I managed it with help from the tutors, reading the forums, and lots of retries). This proved a good way to learn with only slight bouts of frustration, but I wish I’d completed the EdX algebra course before hand as I was pretty rusty on the maths side of things. You get one year of support and must complete the course in that time to qualify for the certificate, otherwise you have to pay extra for an extension. It certainly takes a lot of effort to complete it all in that time especially if you’re working a full time job, but with dedication it is more than possible, and you’ll come away with a good foundations to electronics theory, but very little practical (the fun part!). That’s what building synthesiser is for I guess?