It took a bit longer than expected — I'll admit I was scarred off when I saw the file listing diff — but fortunately the vast majority of the resource files since modified since 1.16 are new, not customized in my pack, or have zero (or practically zero) actual visual changes. I suspect someone at Mojang just forgot and remembered to run a bunch of the images through PNGCrush.
So, my custom resource pack to now updated for 1.18 (with some 1.17 changes along the way), I updated the server.properties with the new URL and SHA sum, and I restarted the server a few minutes ago when nobody was online. So far so good!