KaiOS/GerdaOS webapp compatibility layer?
KaiOS is a quite interesting commercial fork of FirefoxOS and there is a FOSS version of it called GerdaOS (https://gitlab.com/project-pris/system ).
The interesting part is that all apps are webapps like on FirefoxOS, so if the right version of the Gecko engine was made to run on UT, it should be possible to run KaiOS apps on UT.
Given the high number of devices sold with KaiOS in India/Africa etc. there are actually some potentially interesting official apps for it, for example WhatsApp.
And it might be easier to get working than Anbox, given the low resource use of it.
Here is the link of the unofficial community run app-store with a lot of details about it: