I am looking for an affordable hardware device that can be used as a simple but good quality Jamulus client for around 10 non-tech-savvy members of a choir, here in the Netherlands.

A Raspberry Pi 3 (or 4) would mainly run the Jamulus software client and manage the network traffic.

Most important requirements for the audio processing are:

mono input to record singing, dynamic microphone (1/4" or 3.5mm jack or XLR, but NOT USB)
stereo output for headphones (1/4" or 3.5mm jack)
low latency audio processing (10ms or less)
16bit or 24bit depth
48kHz sample rate
low background noise

Optional requirements:

Input gain control for microphone
Output volume control for headphones

Could you advise on a Audio injector board for this purpose that is currently available?

I have a Raspberry Pi 3 available for testing purposes. But a Raspberry Pi 4 would be fine too.

We are in the development of a microphone preamplifier board for our isolated and pro. soundcards. These preamplifier boards will have the option of phantom power as to support condenser microphones, as well as the option to use dynamic microphones.

Until we produce those soundcards, I would suggest using a cheap external microphone preamplifier with any of our soundcards.

If you want headphone outputs, you can use the original stereo soundcard, or the zero soundcard :

Those versions have output headphone gain control in software and input gain control using either the hardware potentiometer or software.

All of these GPIO based I2S soundcards will have similar and the lowest latency for the Pi.

Other features you require can also be implemented as ALSA plusings or by other software means.