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How to get closest to standard linux experience on android 5.1 tablet?
It is an older tablet and was budget at the time. I just want to use it for the massive energy saving compared to my laptop as I am off-grid so very limited power in these low solar periods.
I plan to use it as a desktop, fitting the normal keyboard and other usb devices via micro usb to usb converter.
I calculated I would get a 70% reduction in consumption to use this tablet instead of the laptop so would be a great power saving.
Doing more research around this though it looks extremely difficult to get a full linux OS running, without android at all since the devices are made to be locked into android.
It will not be a supported device for any of the android open source OSes like lineage and such.
There are also linux distro android releases such as debdroid, droidian and mobian, but again these are very restrictive in what devices they support.
Open to suggestions there to be able to have full linux on there but from what I read it will be all but impossible.
The route I am thinking of now is to flash the stock android with an AOSP version, so that it gets rid of the proprietary BS of google - and then use one of the methods of running linux in android from there, which there are several. Apparently they use chroot or similar methods to run linux within android.

Comments
Didn't you start a thread on basically the same topic around 10 days ago?
https://lowendtalk.com/discussion/213884/what-is-the-most-linux-like-experience-i-can-get-using-an-android-tablet
Lol, apparently so. Your memory clearly better than mine.
My two cents:
It's unfortunate in terms of e-waste, but old Android tablets -- especially if not from a major brand -- are fairly locked down in terms of hardware support. Basically, one uses them for as long as feasible/possible as they were intended to be used, and then it's over. (Unless one is very adventurous and willing to make significant compromises)
Lightweight laptops are still the best option for Linux
How do you happen to be off grid?
I have an eMachines e350 laptop whose battery sensors report 7.5W of consumption when browsing the web and 5W when idling with the screen on. These netbooks are under-powered but if you live in a van something like this might be good enough:
I bet those headphones are quite power-saving as well.
Those are firing range earmuffs that I habitually wear indoors like an asylum schizo inmate because of the people of this country. Can also come in handy to someone sleeping in a van on the roadside.
Nice peace of hardware! I've also always wanted to get a netbook. Sadly never got around to ordering one. I mean it's not like those things are expensive or anything.
Repurposing old Android devices seems exciting too but as @angstrom said they are usually quite locked down and require a ton of research/work to get running (if it's possible at all). Hardware support isn't really much of a problem in theory since most drivers are probably available somewhere. It's just that chances are they'll have to be assembled/ported manually.
Install a VNC or RDP client on it, and connect to a VPS or dedi running a full proper OS.
Or if the internet is too poor, then you got some suggestions in another thread that end up basically installing that VNC server with GNU/Linux locally on the tablet itself, instead of a remote host.
Pretty much any old tablet or phone can run some sort of VNC client (bVNC, RealVNC work well, and there's an official RDP client from Microsoft, although I have not used it that much). So, far from e-waste.
The only thing you don't get via VNC is 3D gaming and video viewing, but the latter can be done locally on the device via the Android side of things. Unless a really old device and it cannot do modern HTTPS anymore.
If you're system has a recovery or open oem unlock then you can try PostMarketOS. I tried the x86 version on a mini laptop and it went on in one command. there is an official img and if it doesn't work you can try other people images. It uses alpine which is apks but I managed to get blender on it just for testing and it worked fine.