There are a number of other free software projects to provide a Linux experience for mobile devices. Some of these projects are actively working on moving to Halium. All of them are kindred spirits in the quest for Linux on mobile. Not the least, they have valuable resources available at their respective sites about porting to mobile devices.
LuneOS is based on Open webOS and webOS OSE. It is provided by the WebOS Ports team. The system has it's legacy in projects by Palm, HP and LG. It is a continuation of Palm/HP's webOS that was used in 2009-2011 in various phones and tablets. The project is fully open source and can be build completely from scratch. LuneOS developers are actively contributing to Halium, even though only the minimal Android image from Halium is being used currently. LuneOS actively uses the Yocto Project build system and builds their kernels with GCC provided by Yocto.
Plasma-mobile is based on the Plasma shell, developed by the KDE community. Plasma mobile is available on Halium.
PostmarketOS is based on Alpine Linux. It has a selection of multiple phone interfaces. A main difference to Halium is that postmarketOS targets upstream kernels. This brings the advantage of more up to date software. It has the disadvantage that device drivers which are only available as binaries for old kernels can not be used.
Sailfish OS is based on the Mer Project. The user interface is developed by Jolla and not open source. The technical architecture should be largely compatible with Halium.
Ubuntu Touch is based on Unity 8 and is developed by UBports. It is the continuation of the earlier Ubuntu Touch project from Canonical. UBports is actively working on making their future releases available on Halium.
Droidian is a GNU/Linux distribution based on top of Mobian, a Debian-based distribution for mobile devices. The goal of Droidian is to be able to run Mobian on Android phones.
There are more projects that share similar goals and/or similar tools and might have valuable resources. A few shout outs: LineageOS, MaruOS, Neo900, Purism Librem 5, Replicant, Tizen.