...and here I am, running a blog that if it gets 15k hits a second, it won't even bat an eye, and I could run it on a potato. Probably because I don't serve hundreds of megabytes of garbage to visitors. (The preview image is also controllable iirc, so just, like, set it to something reasonably sized.)
Meson and CMake are the two major players I've seen along autotools. Are they better? In some respects, yes (especially Meson, imo), in others... not really. For a pet project that only targets two platforms, I'd just stick to handwritten worst-practices Makefile. You will likely have less trouble with that than any of the others, simply because you know it already.