There is no Debian package for mythtv, so I had to build from source.
The Raspberry mythtv frontend is running version 1.32. Apparently the frontend(s) and backends should be running the same version, so I built version 1.32, instead of the latest version 1.35.
Main references are:
Used the following steps (as root) to build a local repository of mythtv packages:
apt install git ansible cd ~ mkdir mythtv cd mythtv git clone https://github.com/MythTV/packaging.git -b fixes/32 cd packaging/deb export root="" # workaround for bug in build-debs.sh ./build-debs.sh fixes/32 dpkg-scanpackages -m . > Packages # Created file /etc/apt/sources.list.d/mythtv.list: deb [trusted=yes] file:/root/mythtv/packaging/deb ./ apt update apt install mythtv-backend-master mythweb mythtv-frontend
Added user fog to the mythtv group.
Ran mythtv-setup to check configuration - seemed to have retained settings from Ubuntu OK.
Corrected mythtv password in /etc/apache2/sites-available/sites-available/ and /etc/mythtv/config.xml. Installing mythtv had set these to a random string, preventing both the web interface and the backend from accessing the database.
Downloaded dvb-demod-si2168-b40-01.fw firmware and put in /lib/firmware.