(But I
could use the binaries from TL as a starting point.)
well, something that works is better than nothing but of course one can always install a scheme from texlive.
That's either cross-compiling, but probably easier to simply put some
proper hardware in place, so that we can also run the test suite.
I was thinking of getting hold of another Raspberry and trying to
install a 64-bit system on it. Or maybe dig out my super old Pine64
and build on that one.
ubuntu 64bit is ok on rpi3 & rpi3b+
You can try A64 form olimex, also ok with ubuntu 64bit.
And of course the rpi4.