Just to help others reading this, could you say a little about the process of opening it up & ultimately installing TWRP & LineageOS.I did end up having to open it up. (twice) Didn't install Majisk or new ROM the first time. I re-opened it and have Linage 12.1 installed now. GApps failed to install for some reason. I'm going to install the fire toolbox and try to install them that way. Thanks for all of the help!
I found LineageOS 12 to be faster & with less issues than LineageOS 14. Eventually I gave up on both of them & went back to FireOS (but rooted & with TWRP) just so I could use it as an Alexa device with hot-word detection. Am I right in thinking there is an experimental LineageOS 16 for the FireHD10 2017 (suez)?
EDIT: There is a LineageOS 16 (see here) but it seems to have quite a few unresolved issues including camera & headphones.
I think Gapps Pico or Nano are the versions to use with LineageOS.