I can use an existing account (already attached to a phone number) so the device doesn’t require a SIM

The device never requires a SIM to actually use Signal/Molly, you only need to enter the verification code when signing up, it doesn't matter on which device you receive the code.

Unfortunately at the moment I am having to use “Molly” not “Molly FOSS”, meaning I have some of the proprietary blobs in normal Signal app. It is currently too much hassle (for me) to setup notifications with the FOSS version.

It's very easy, you don't really have to set up anything, it just uses a little more battery life. If you already use multiple apps that support UnifiedPush, you can check out Molly-UP. It's available in the Molly F-Droid repo, or you can just pull the apk from GitHub using Obtainium. You don't have to self-host anything, there are public instances of both mollysocket (like molly.adminforge.de) and ntfy (like ntfy.rollenspiel.monster or ntfy.adminforge.de).