I take it this won't work with tectonic due to how it widens rivers so much? I tried it out in a new 1.21.1 world and flew around for a bit but no luck. Also couldn't figure out how to use /locate command to help.

I'm playing Larion with this mod, and I also have not found any bridges yet; even across relatively small channels of water. But perhaps it's the biomes also. I'm using William Wythers' Overhauled Overworld and Expanded Ecosphere, and maybe there are new river biomes that aren't supported or something.

Note that in the mod description of Terralith, it says "Bridges may still spawn but will be rare". So my guess is you're correct and the rivers are too wide to add bridges to in Tectonic and Larion. I think the statement "YUNG's Bridges is intended to be fully compatible with every worldgen mod" is a confusing overstatement.