I haven't tried this myself, but someone made a python script for the signature: https://forums.developer.nvidia.com/t/tegrasign-replacement-for-jetson-nano-for-dtb-file-generation/83076
This one is specially for DTB, but if I understand it correctly, for U-Boot you'd only need to change the partition "magic", i.e. change DTB to LNX: https://gist.github.com/TIS-Arne/879f47a0310bb46ae426f1b8bb98e5f9#file-mytegrasign-py-L36
In L4T 4.6, by default, they put everything to the SPI except rootfs, which is on sdcard. However you're free to pick whatever schema you want (just open the XML files and follow what they're doing), you can even put just u-boot to sdcard.
I myself am happy with Nano as SBC (it's more powerful that Raspberry Pi 4, has PCI Express, is quite stable as a home server), though yeah the upstream status is still ongoing. They should release Jetpack 5.0 any day for the other boards, hopefully the GPU driver for 5.10 kernel is going to be more upstream compatible.
