Is it crazy to save all my raw files to disc as back-up and import as DNG to Lightroom?
No, it's not crazy, what what exactly are you hoping to accomplish with this? For long term storage, DNG makes a lot of sense. But short term it may complicate things a bit in workflow and backup. Again it depends on your needs...
One thing you should not about DNG is that all of the xmp metadata is stored in the DNG and not in .xmp sidecar files. This can be good or bad depending...yes, it locks all the metadata inside of the DNG and not in a loose sidecar file. But if you are backing up you images using a backup software based on file modification dates, a modified DNG will pop a new modification date and require the backup of the entire DNG file–which can be quite large compared to backing up a tiny .xmp text file. So, if you are using Lightroom and saving all the metadata into the DNG file, those changes force the whole file to be backed up.
I import the original native raw files in Lightroom to work on them and save out the .xmp metadata. At the point where I think I'm "done" with an image and want to embed all the metadata inside of a DNG, I'll convert to DNG from inside of Lightroom...this is done on a project by project basis...
So, you really need to think about your workflow and decide what it is you are expecting to accomplish.