Most Android systems provide possibilities to add sdhc cards. IMHO the most versatile option would provide a minimal patch to the built-in bootloader which would check if Grub is installed on that card.
When not, it would continue booting the standard built-in Android system, otherwise Grub would be started and this allows to select either the old built-in system or any other system available on sdhc card.
I have this system on PinePhone and would very much like to have it on all my phones.
Hi @tricky1 welcome to /e/ land.
I am not quite sure if this is likely to be the direction of travel for /e/ right now, but I for one would be very interested to know where ideas like your proposal are being discussed - and how to incorporate dual boot into my device. Thanks in advance.
It’s a sad story that, although the number of users hating Android is high, the number of easy ways to deGoogle it is close to zero.
I’d like to gather some enthusiasts realizing my wish of adding /e/ in the sd-card.
Great, but please provide some active links, so that we know where to gather! 
It is interesting. I drank deep of the Google Kool aid and still need the goog on my daily driver. I also enjoy watching time on the facetoobs.
I have installed Shelter and put Facebook there and looking to try that with Google on an /e/ phone. It’s nice because it separates those apps from the rest of the phone and you can freeze them when you aren’t using them. The downside is getting photos into the shelter world.