AFAIK there are no Chrome uBlock alternatives, while there is a Chrome alternative with uBlock, Firefox. Beside that, you can set

    "ExtensionManifestV2Availability" = 3;

as see https://chromeenterprise.google/policies/ apparently till June 2025 to keep Manifest v2 extensions like uBlock fully working, I've rebuilt my NixOS with Chromium this morning and uBlock was there so it's not removed at least if you have the aforementioned option set.