Looking forward to your template. Don’t forget musl.
I disagree with @oliver about /opt for any packages installed by xbps. However I’m no expert on Void templates or the FHS. I don’t know whether I can help. What I would say is that since the author gives you freedom with his file tree, there is no compelling reason to use nonstandard locations.
I recall node.js can go anywhere, as many forks and clones as you please, each app with its own node.js support folder. I don’t know what Void considers “canonical” location(s) for node.js, if any, but I don’t recall that node.js actually has one anyway. It’s “portable” as they say. If it were me I’d probably dump the supporting node.js “world” folder into /usr/bin right alongside the invoking shell script of whatever kind works for Tagstoo.
Tagstoo binary release has 3 shared libs (libffmpeg.so, libnode.so, and libnw.so) and its source shows beaucoup node.js packages. So I guess the deps are ffmpeg, node.js, whatever provides libnw.so, plus all the many node.js packages.