AFAIK some of the other chromium forks (brave and/or edge?) were committed to backporting manifest v2 (or more specifically the webRequestBlocking API) for future chromium versions.
Brave has integrated uBO directly into their core logic. Visit brave://settings/extensions/v2 and you can download it, even with no MV2 support. I'm not aware of any other browser adopting this approach.
firefox 149+ actually bundles brave's adblock-rust as well, it just doesn't really use it. waterfox does enable it and use it, though they're still testing (https://github.com/BrowserWorks/waterfox/issues/4182).
all of that being said, I was answering the question about mv2 broadly, not ad blocking.
This seems to be the only way forward from what I can figure. Helium's main selling point is that it's essentially degoogled chromium + a few miscellaneous patches & full uBlock. But once Google completely strips all that out of Chromium project, that won't be a tenable option.
I'm not sure what Opera/Vivaldi/et al. use for their native adblocking, but Brave's rust adblocker makes the most sense to me. Really it's uBlock's filtering lists that keep the whole thing working anyway.