Sounds like Chrome isn’t willing to support extensions on mobile, but Edge is and accepted Arnaud’s Kiwi contributions into their codebase.

It’s just as well, Kiwi Browser wasn’t actively developed beyond its original features, it basically just re-added extension code into the codebase, and each update just fixed git conflicts between that patchset and the latest updates to Chromium’s codebase.

From the article:

It also doesn’t seem like extension support in Microsoft’s browser is as straightforward as Kiwi, requiring you to paste the extension’s ID (e.g. “cnlefmmeadmemmdciolhbnfeacpdfbkd” for Grammarly) into the ‘Extension install by id’ field. By contrast, Kiwi lets you install add-ons directly from the Chrome Web Store or via a downloaded file.

Edge’s extension support is buried behind a bunch of manual steps, most of which the article didn’t mention, and installing each extension is arduous. The only mobile browser left that has full extension support is Firefox.

If I just upgrade to the latest version (by flashing) and not install mind the gapps, is it basically a degoogled phone?

It would probably have to be a clean install that wipes userdata, not an upgrade. But yes, LineageOS is pretty much as degoogled as you can get on an OS forked from Google’s AOSIP, if you don’t explicitly add GApps.

German privacy researcher Mike Kuketz does not agree, but it all depends on how much effort you’re willing to put into degoogling, or how much you’re willing to compromise on usability.