FWIW, all my systems have been upgraded a while ago without removals....I'm pretty sure the qt5 transition is done: https://release.debian.org/transitions/
apt-get -o Debug::pkgProblemResolver=yes dist-upgrade
The problem appears to be that I need akonadi-backend-sqlite 4:18.08.1-1+b2 but apt can't find it in sid, although it is the only package apparently available!
# apt policy akonadi-backend-sqliteakonadi-backend-sqlite: Installed: 4:18.08.1-1+b2 Candidate: 4:18.08.1-1+b2 Version table: *** 4:18.08.1-1+b2 500 500 http://httpredir.debian.org/debian sid/main amd64 Packages 500 http://httpredir.debian.org/debian testing/main amd64 Packages 100 /var/lib/dpkg/status
apt policy akonadi-backend-sqliteakonadi-backend-sqlite: Installiert: 4:18.08.1-1+b2 Installationskandidat: 4:18.08.1-1+b2 Versionstabelle: *** 4:18.08.1-1+b2 500 500 http://httpredir.debian.org/debian unstable/main amd64 Packages 100 /var/lib/dpkg/status
cat /etc/apt/sources.list.d/debian.list # debian loadbalancerdeb http://httpredir.debian.org/debian/ unstable main contrib non-free#deb-src http://http.debian.net/debian/ unstable main contrib non-free#testing#deb http://httpredir.debian.org/debian/ testing main contrib non-free# Experimental#deb http://ftp.us.debian.org/debian/ experimental main contrib non-free#deb-src http://ftp.us.debian.org/debian/ experimental main contrib non-free# stable#deb http://ftp.nl.debian.org/debian/ stable main contrib non-free