many years ago I began looking at the idea of tablets as a good compromise of the various factors you bring up, and two years ago before a two month trip, I ended up getting a smaller tablet that could fit into my handlebar bag (a regular sized ortlieb) with the protective flip case attached.
The screen size is a bit under 8 inches, and with the case, its about 8.5x5.5

I looked at various tablets, and the priorities were size and reasonable functionality. I went with a very moderately priced Samsung android model, around 200 bucks, as I was concerned about worst case scenarios-theft-and also didnt want to spend that much anyway and in the end, this level of product was both well constructed and performed well enough--it would have been nicer to have a faster processor for working on photos, but I had no interest to spend 800 bucks on a tablet and worry about being held up at gunpoint.

like all things in life doo dads wise, its a compromise, and this size tablet is fine for watching stuff, using a maps app and built in gps if you want to check where you are (which I did on occasion) when paper maps were confusing, or to situate myself in a city or whatever that was complex.
It has a slot for micro sd cards for extra storage, but I used this to transfer photos from the sd/micro sd adapter in my camera at the end of each day, then would work on my days photos and write up my trip journal. Works ok, but again, limitations on not having a keyboard, although portable keyboards are available.
I was travelling with access to plugs in cheap hotels, so charging wasnt an issue, and I wasnt using it during the day, so worked out fine even without a battery bank thing.

Being able to fit in my hbag was important, and I am still using this tablet daily two years on and it is well built enough that its in great shape. Bonus for me being android is that I use an android phone, so didnt have to learn how to use it. I have been a life long mac user, but find android to be intuitive and easy to use, although you would probably find a ipad easier and a no brainer to use, not to mention easier interactions between ipad and mac computers if you use them.

re kindle--yes, it makes sense that they are a better reader, great battery life. I personally know that on bike trips I dont usually have the energy to read much, especially if working on photos and writing a journal, but it does make sense that they are a great option, and one you may consider also if this is important to you. It is however, more weight, phone, tablet, kindle....adds up...but the priority is for you to make. And anyway, you perhaps have a tent that weighs a few lbs less than the tents I carry on bike trips, so it can be a moot point anyway weight wise.

in the end, I found a tablet to work just as I thought it would and I was very glad I got it and used it on my last three trips.
Its simply a tool and I found the bonus of entertainment, help with OSM mapping offline, browsing the web for route planning etc, and a small compact photo editing and writing and skyping or whatever tool, to be a great compromise of size, weight and functionality.