This turned out to be very simple.
In addition to Signal on my phone, and the Signal Desktop app on my computer, I can now have Molly on my tablet. Not crucial for receiving messages, but very useful for sharing links/messages while browsing on the tablet.

1. Add Molly's repo to F-Droid (url: https://molly.im/fdroid/foss/fdroid/repo).
2. Install Molly.
3. Select "Link existing device" in Molly and then use Signal's Link Device function from the phone.

This tablet lacks any Google services, and in the absence of a dedicated MollySocket (which I have not bothered to setup, maybe if I decide to use Molly instead of the Signal client on my phone) the app uses WebSocket which is power-consuming but that does not bother me on the tablet.

• https://github.com/mollyim/mollyim-android
• https://molly.im/fdroid
• https://lowtek.ca/roo/2024/signal-desktop-on-an-android-tablet
• https://xn--gckvb8fzb.com/if-you-must-use-signal-use-molly