thank you all for your help!
@kovid - i did not realize that emailing to the kindle address automatically converts epub files to azw3!
@DNSB - i am big dumb. more googling helped me find the section in preferences -> behavior & preferences -> plugins -> device interface where i can have different formats set up to send to different devices. it now makes a great deal more sense why you can have multiple formats of a book in your library at one time. lol
i had no idea this was in here?? calibre is so huge i keep finding something new to customize even after weeks of poking at it. i thought i'd have to go in and manually change the export format type back and forth every time i switch between sending things to my kindle v.s. my tablet since the tablet can't do anything with an azw3 file. i'm not concerned with space since i have several TBs on this computer...it was more an issue with my own computer competency/remembering to switch between formats every time!
i am now set up to usb transfer converted azw3 files to my kindle, and the original epub files to my tablet. god bless calibre customization options.
sidenote: GMX granted me access to my account again today and apologized for the inconvienence and said they're working to "resolve the issue". i have no idea what this means, if they were flagging me for spamming. you'd think they'd be more "stop doing that" about it all.