So after last weekend's experiment using FreeBSD as a router OS again, and learning that almost all my pf(4) knowledge has gone out the window lately, I decided to have a crack at installing Alpine Linux. I did an even worse job documenting my progress here, but I'll try get something together as documentation for this is entirely nonexistent.
For reasons unknown to me, Alpine uses vfat filesystems for removable media like SD cards. Once I got around (or rather, merely understood) that, it was fairly easy to use
setup-bootable to write the image to an SD card using my desktop, edit syslinux.cfg to enable the serial console (with appropriate baud rates), then chuck it into my APU with the serial cable connected and PuTTY running.
From there, it was fairly trivial to get simple NAT running, with DHCP and BIND for DNS - I spent Sunday putting the finishing touches on Dynamic DNS for the two zones for my home network. There's lots to like - it's a tiny bit cooler again under normal operation than FreeBSD (which was substantially cooler than pfSense), in entirely unscientific testing. I definitely dig the fact that I can yank the power at any point and the system is unlikely to shit itself - I could probably have reproduced that by implementing NanoBSD, and I may still do that if I find myself bored one weekend. This particular behaviour is apparently going away in future versions of pfSense though (but they will not support my aging hardware anyway).
lbu (local backup) makes things super easy once I've made changes to the configuration though, and it's not that difficult a step to remember after changing something. Better still, my power-off outage time is super fast: ~35 seconds from boot to having network connectivity - and most of that is spent in the APU's BIOS (I'll have to investigate if it's possible to cut that time down). I'm pretty sure after a power outage my VDSL will not negotiate a connection that fast, so we'll be back online with the absolute minimal delay (of course if I really cared, I'd invest in UPSes).