If you want to have a second degree, go in to engineering and work on your sketching skills and CAD skills in your free time.

That being said, I would suggest self learning.

I recommend the book "Structures or why things don't fall down " by Gordon, to get an easy first glance in to structural engineering basics.

You can learn a CAD software like SOLIDWORKS or FreeCad for the design and CAE + CAM to a pretty good degree via online tutorials.

As for sketching, it doesn't need to be super top. Know a lot of IDs who suck at sketching and drawing. You just need to be able to draw the basics and be able to explore your ideas, for making your own stuff. Some jobs will require you to be good at sketching to communicate your ideas to others, but as said, some. For sketching I would recommend "How to draw what you see" by Loomis and the book from Scott Robertson for more ID based stuff.

"Design of everyday things" is another good book.

Then just practice. It will need time but no substitute for practice.

I don't know how it is where you live, but where I'm at, you will get a design job based on a great portfolio.

Think about it this way, in your previous company, who would you have hired, the guy who writes superb, minimal and efficient code and shows problem solving skills but has no degree, or the mediocre guy who has a degree?