I haven't tried on an A64, but CPU mining Monero on a Rockpro64 can easily be done by running the
armv8 build of monerod + compiling and running
xmrig.
(I went to a lot of trouble compiling monerod from source before realizing that the precompiled linux armv8 binary on getmonero.org just works. Monerod is a huge project that takes several hours and a lot of RAM to build on rockpro; xmrig is small & quick to build.)
Rockpro64 with 4GB RAM costs $80 for the bare board and maxes out at 170 RandomX hashes per second, at which it runs quite hot in the fanless Pine64 aluminum casing - about 135F in a room at 64F ambient. That isn't hot enough to damage most hardware, but NAND flash wears out faster at higher temperatures, so if running it with an emmc the temperature becomes a concern. Using an external SSD isn't really worth it - it will sync faster, but it syncs fast enough anyway, and it doesn't help with mining. monerod itself can mine at around 140 H/s; xmrig with 100% hugepages is more efficient and gets 170 H/s.
That isn't enough to be worth running on mainnet at all; I ran it a bit on stagenet for fun.
Meanwhile an AMD Ryzen 9 5950X CPU gets around 21.5 kH/s, or 126x faster, and costs $745, or 9x as much. You also need to calculate the hashes per kilowatt-hour. My impression is that SBCs are useful if you need a lot of little computers but they aren't economical for building supercomputers with.
Clusterboards, a.k.a. bladed servers, are more for building things like web servers that benefit from having many copies of the I/O hardware.
For a standalone monero node that runs all the time, you want it to be as physically small as possible, and an SSD doubles the size. The daemon contains embedded proof-of-work hashes that let it sync up very quickly to the blockchain height at the time it was released. After that, 1 month's catch-up syncing took ~10h on a rockpro with the blockchain on emmc, which is fast enough for something you're only going to do once.