There's zero support for Amlogic, Allwinner (H6 and older) and Rockchip (mainly RK3399) have best support available when it comes to "cheap" SBC.
There's preliminary support for Pine64 Quartz64 in -CURRENT but it's far from complete. Given your requirements your best bet would be RockPi4 or Pine64's RockPro64 however due to hardware limitations 4Gbyte RAM is max. They're doable buildboxes but it will take time(tm) and you have to be careful with ports that consumes a lot of memory such as llvm (flang) or GCC. Allwinner works well however they're limited to 3Gbyte or less despite having 4Gbyte in some cases. There is only official support for mainline u-boot , if you need to use forks etc you're on your own unless/til you're willing to submit upstream. Scan the ports tree for u-boot ports and you'll pretty much see what's supported.
For Rockchip you also have https://wiki.freebsd.org/arm/RockChip
If you want beefier ARM64 hardware Ten64 and/or Solid-Run have some offerings that are quite well supported.
There's preliminary support for Pine64 Quartz64 in -CURRENT but it's far from complete. Given your requirements your best bet would be RockPi4 or Pine64's RockPro64 however due to hardware limitations 4Gbyte RAM is max. They're doable buildboxes but it will take time(tm) and you have to be careful with ports that consumes a lot of memory such as llvm (flang) or GCC. Allwinner works well however they're limited to 3Gbyte or less despite having 4Gbyte in some cases. There is only official support for mainline u-boot , if you need to use forks etc you're on your own unless/til you're willing to submit upstream. Scan the ports tree for u-boot ports and you'll pretty much see what's supported.
For Rockchip you also have https://wiki.freebsd.org/arm/RockChip
If you want beefier ARM64 hardware Ten64 and/or Solid-Run have some offerings that are quite well supported.