Debian/Raspbian install QGIS 3.x

Re-posted from GIS stackexchange as this is becoming more of a software question.

I'm trying to install QGIS 3.x on Raspbian Stretch with Desktop.

I found this build-from-source guide using this forum post, but my linux-fu is weak.

I followed sections 3.5 and 3.6 of that guide, and then started on section 3.8.

At the dpkg-buildpackage -us -uc -b stage, I get an "unmet build dependencies" error. When trying to sudo apt-get install the missing packages I get several "Unable to locate package" errors (libqscintilla2-qt5-dev, qt3d-assimpsceneimport-plugin, qt3d-defaultgeometryloader-plugin, and qt3d-scene2d-plugin).

I've already added deb https://qgis.org/debian unstable main and deb-src https://qgis.org/debian unstable main to my sources.list file (as per QGIS install page...but now I'm stumped as to what to do.

Do I need to add another source (which one?) to that file, or is it something completely different?

Solution

Alternative to building from source.

Issue: with the QGIS install instructions (assuming I followed them correctly) - it was unable to find the python3-qgis package.

Solution: basically use a different mirror than what the install recommends.

Starting from a fresh Raspbian Stretch with Desktop install.

Run: sudo apt-get update

Run: sudo nano /etc/apt/sources.list

Uncomment the deb-src line (not sure this matters).
Add the following line: "deb http://ftp.debian.org/debian sid main" (alternative mirrors can be found here).

Run: sudo apt-get update

Run: sudo apt-get install qgis python3-qgis qgis-plugin-grass

This will install ~300 packages, and can take a while (~4min to download for me). After the download there is a changelog/notice that must be read (keep pressing return then q). Soon after starting the download there is a prompt...once you're past that you should be safe to step away while it installs.

Once done you can launch QGIS (3.4 Madeira LTR as of this writing) by simply typing qgis from the terminal.