So having discovered a bug in the Qgis composer as the reason my homebuilt Qgis VMs crash and not any other reason, this idea of building from source has actually been not such a bad thing at all and has left me with something that actually works all in one VM and one that is actually licensed legit because I don’t actually own any Windows licenses and haven’t much like relying on some legacy licensing rights to Windows environments that I will sooner or later have to update to current.
This being achieved by being able to build what seems to be the last build (313ec55) of the Qgis development master that is actually reliable, onto versions of both Xubuntu and Debian that can run a release of Qt that doesn’t have a silly floating point rendering bug that never existed until quite recently and as it happens was the version deployed with Xubuntu 16.04 which is the last version that can auto install 313ec55 from the packages without installing the latest version, so building from source on a later version of either Xubuntu, or else on a late version of Debian, was the only way of getting this 313ec55 fairly stable with few bugs version 2.99 of Qgis that I can use to develop my 2.99 map projects and use for virtually every part of the process without issues. At the moment there is still a bug that relates to saving new projects or new versions of a project that needs the use of Windows because the 2.99 project format is incompatible with 2.18 in which this particular bug has been fixed.
The development masters for 2.99 are of course part of the process of developing the new 3.0 release of Qgis and I guess there will soon be a 3.0 version out, probably in a couple of months, and hopefully all the bugs will have been fixed in it.
However a big discouragement is that the latest builds have introduced an incompatible project format and the developers have said they do not care that their new software is incapable of reading and converting older 2.99 project formats because they don’t guarantee to support projects created with previous development versions of the software. This will have to be followed up to see if this is a new policy and is it reasonable.
In general as far as the VMs go both Debian 9.1 and Xubuntu 17.10 are very efficient with resources. A 12 GB VM will have loaded Qgis, but not a project, and have used up 1 GB of memory in total for the OS and the software. This is a lot better than Windows bloatware where the OS gobbles up memory like there’s no tomorrow. You can build a really lean VM or PC optimised for low resource use and that’s where a GUI environment that is optimised for low resource use like Xfce (which is what I have running with Debian on my Qgis build VMs) really shines, one of the reasons I stopped using Linux Mint (which I have almost forgotten) because their GUI gobbles resources unnecessarily.
Meanwhile the map work continues.