I'm no help here, but I just wanted to say I pretty much agree with your last paragraph, above. In case I do have to replace my 1-1/2 year old Forma, I'd probably have to go with a Sage. But, I've been looking around and the Boox Leaf 2 looks interesting:
https://youtu.be/sCTDcmUfpfI
BTW: How did you open your Forma? Is there a video somewhere? Does its waterproofing survive? Could you upload or link to a photo showing the innards (and especially the battery)?