The
Quartz64 Wiki is a good source for what software is available. I have tried with success Manjaro, DietPi, and some Balbes150 supplied images like Armbian and FreeBSD. Balbes Armbian is the easiest to install for Debian forks since you write the image to disk and after a first boot initialization it is running, however DietPi gives you a ton of configuration options during installation allowing for a desktop workstation, various servers, or media center PC. After taking some time with the DietPi installation, I prefer it for a workstation but this is a preference and not a reflection for how good both OSes are.
Compared to Raspberry Pi ecosystem, Pine64 products are not as user friendly in the sense that Pine64 supplies the hardware but you are entirely reliant on 3rd parties for software, with some hardware having better support than others. Due to a partnership with Manjaro, generally most Pine64 SBCs that are general purpose computers enjoy more formal support than other OSes. I am a Debian fan so usually work with Debian forks, including Raspberry Pi OS, Armbian, and Diet Pi.
When evaluating SBC vendors, especially Raspberry Pi alternatives, consider the system on a chip (SoC) adaptation over the SBC since the more popular the SoC is aross SBCs, the more likely OSes support it. The RK3399 powering the RockPro64 is widely adopted so there is some excellent support for all of its features. The RK335x family is more powerful but not as popular at the moment, consequently giving the Quartz64 limited support. The RK3388 SoC is proving to be very popular but Pine64 doesn't have an SBC available with it at the moment. Raspberry Pi's popularity gives it a wide variety of alternative OSes to Raspberry OS (or whatever they call it).
Whatever SBC you choose to purchase, consider having serial console via UART access to it for debugging purposes. For Pine64, most of the current SBCs need a 3.3v USB to UART adapter capable of up to 1.5M baud, like their "woodpecker" adapter, though 3rd party options also work. The Raspberry Pi family usually needs only 115200 baud, which is the most common speed. It is the characteristics of the SoC that determines baud rate, not the SBC manufacturer, usually. If you purchase an eMMC module for your SBC, also get the USB eMMC writer module, it can be a pain in the butt to try to program the eMMC module from a microSD card. The eMMC modules are also incredibly quicker than using the microSD card. I haven't booted my Quartz64 from the onboard SATA yet.
Quartz64, RockPro64, PinePhone Mobian, PineBook Pro, PineTime, and all the trimmings that make FOSS fun.