Just FYI -- I am in the process of creating code to build calibre and all its dependencies on all three platforms automatically. It uses docker for the linux builds and VMs with just the development tools installed for the other OSes.
https://github.com/kovidgoyal/build-calibre
You could fork the repo and use it to create Sigil packages quite easily. You just need to replace the linux/freeze.py script with something that creates a sigil bundle instead, with all the dependencies already built and taken care of by the repo. About 500 lines of python code, though for Sigil the replacement will likely be smaller, since Sigil is much less complex than calibre.
You'd also need to make to change the scripts used to build python to build python 3 and Qt to include those components of Qt that Sigil needs, but that should be about it.
Note that this is not intended to pressure you to create Sigil packages, just to point out a way you could piggyback on my work, to reduce the burden of maintenance, if you decide to do it in the future.