I am currently working as an optical designer. Where I'm from, there is no local institution offering a course in optical design, so most of us here are, to an extent, self taught and/or trained on the job. I only decided on this change of direction a couple of years ago. I have engineering background but also did not know where to start, so quite a bit of time in the beginning was spent floundering around through random books and videos.
If you get your math/trig skills brushed up slightly, something you can look into is the First Order Optical Design course on Coursera, presented by University of Colorado, Boulder. There are three courses making up a specialization in optical design, this is the first one. The second course gets into interesting but tricky physics and math you probably won't ever need for what you want to do. But the content of the first course may be a good starting point for you.
The course includes a design task, which took way more time (for me, anyway) to complete than their time budget suggested. So, perhaps, skip the tests and assignments - Coursera has an "audit" option for some courses that gives you access to the content but doesn't enroll you for the course.
Your experience may differ, but this was a good starting point for me insofar basics of imaging and simple system design go.
https://www.coursera.org/learn/first-order-optical-system-design
Another thought - lens elements in a given system are usually optimized for that system, to work in conjunction with the other lens elements in that same system. So repurposing will work, but you will likely end up with systems that aren't as good as they could be. Just something to keep in mind when trying to figure out "why I can't get a sharper image out of this thing I built from existing lenses". But I'm guessing you're not aiming for high performance systems right now, so don't worry about this for now.
- Sorry for the long answer. And I'm by no means an expert - gots much to learn still. Have done one or two designs, though.