Note knowing what you already know, I;d suggest starting right at the beginning with DC and then AC circuits, move into semiconductors and then take Faringdon advice and start building circuits you are interested in. Let face it, keeping focus is easy on stuff you have a personal interest in! Beyond that you should already have most of the math and physics to help you understand concepts in electronics. You might think it is too early to start designing anything and I woould say yes if you take anything literally, however you should have little trouble with transistor and opamp circuits at least at the introductory level.
Be aware that there are different levels or depths to what is available on line, some of which is more suitable to preparing technicians as opposed to engineers. Usually there is a massive difference in the math required. However don't dismiss one or another, as sometimes that ability to learn something depends greatly upon how the matter is delivered and you are already conversant in higher level math. I.E. some teachers are better than others.
Learn to program. This might not be important for the creation of analog systems but it is critical for digital systems. Frankly this is where good formal training is likely to be more worthwhile to you because it is so easy to become a bad programmer. Here I'm specifically saying that you need better programming skills than you might have received in your mechanical engineering course. Beyond that well developed programming skills can even help in the analog world for data collection and automated test.
When you say systematic to me that implies college level courses that follow a rigorous step by step process. So look for programs that emulate the progression that college level courses follow. While not a replacement for engineering the Navy's NEETS course on line is probably one of the better FREE sources of technical education for technician level electronics, so a good free resource for everybody. One big problem is the rapid change in the world of electronics, which brings up probably the most important element, the ability to read datasheets and tech notes from the manufactures. You can come across technology that isn't really covered well in the first year or two of college level education, so you need to reference the data sheets.
Learn to use spreadsheet software and database management software if you haven't already. To a deeper level learn the ins and outs of the so called "power user" software supplied/available with your OS of choice. This includes things like terminal / command line access to the OS, Python (hopefully supplied with the OS or installed yourself), terminal emulators / communications software, and other goodies. Install a good text editor. You might think that this is going overboard but there is a lot of stuff that is supplied by manufactures in spreadsheet form, or compressed in various forms that you will want access to. A good text editor can allow looking at those files without messing them up. Further I've notice that many engineers graduate from school not really having a good grasp on spreadsheets which are pervasive in industry, it doesn't hurt to take a deep dive with a good book here. I know I'm really going off topic here but spreadsheet forms are used for just about everything in some organizations.
In any event dig in and have some fun. Your background should easily let you step into simple transistor and opamp circuits with in weeks of starting a program. Start designing early into your educational process (whatever you choose) even if you need to leverage existing designs as a guide.