Raising the buffer duration to 8 000ms didn't do a whole lot. But raising it to 30 000ms, and also lowering the low watermark to 0.10 has almost fixed the issue. There is now just the occasional skip (e.g. once every LP play).

Isn't it possible to implement Subsonic in such a way that it buffers the entire song? This is how Sublime works, it actually buffers the playing song plus the two next tracks in the play queue.

@jonas
HTTP/2 was already disabled.
Radio streams seems to work, but have not tested extensively.
Package list:

ii  gstreamer1.0-alsa:amd64               1.18.4-2         amd64        GStreamer plugin for ALSA
ii  gstreamer1.0-gtk3:amd64               1.18.4-2+deb11u1 amd64        GStreamer plugin for GTK+3
ii  gstreamer1.0-libav:amd64              1.18.4-3         amd64        ffmpeg plugin for GStreamer
ii  gstreamer1.0-plugins-bad:amd64        1.18.4-3         amd64        GStreamer plugins from the "bad" set
ii  gstreamer1.0-plugins-base:amd64       1.18.4-2         amd64        GStreamer plugins from the "base" set
ii  gstreamer1.0-plugins-good:amd64       1.18.4-2+deb11u1 amd64        GStreamer plugins from the "good" set
ii  gstreamer1.0-plugins-ugly:amd64       1.18.4-2         amd64        GStreamer plugins from the "ugly" set
ii  gstreamer1.0-pulseaudio:amd64         1.18.4-2+deb11u1 amd64        GStreamer plugin for PulseAudio (transitional package)
ii  libgstreamer-gl1.0-0:amd64           1.18.4-2     amd64        GStreamer GL libraries
ii  libgstreamer-plugins-bad1.0-0:amd64  1.18.4-3     amd64        GStreamer libraries from the "bad" set
ii  libgstreamer-plugins-base1.0-0:amd64 1.18.4-2     amd64        GStreamer libraries from the "base" set
ii  libgstreamer1.0-0:amd64              1.18.4-2.1   amd64        Core GStreamer libraries and elements
ii  libsoup-gnome2.4-1:amd64 2.72.0-2     amd64        HTTP library implementation in C -- GNOME support library
ii  libsoup2.4-1:amd64       2.72.0-2     amd64        HTTP library implementation in C -- Shared library

Playing back audio in Strawberry from my Subsonic server results in intermittent skipping/stuttering. Playback using the Sublime player on the same computer from the same source has no issues.
The server is hosting FLAC files, with no transcoding on the server side. Are there any configuration options available regarding buffering on the Subsonic implementation in Strawberry?

My setup
Client side:
Strawberry 1.0.15
Pulseaudio 15.0

Server side:
Airsonic 10.6.2