I’m afraid that if you don’t have a tenant that can perform search over custom metadata fields when using search engine v2 then you won’t be able to accomplish that in the extension. As noted in some of the links you posted this ability it’s not available to every tenant.
As a personal opinion, soon in software development tends to be a bit more of an unknown then the dictionary definition. What I know is that at this precise time the v3 option is still not generally available so forcing DAE to use it (which would be a quick thing) is not feasible at it would be broken for some. In addition, there are some changes between both engines so even when v2 is deprecated there’s still need to be some time for people to move out of it. With all these in mind I’m a bit unsure in terms of timelines for the DAE. I’ll relay this information internally so that at least I know the possibility to use v3 in the extension is already being tracked.
I just wanted to provide you with yet to be confirmed, but possibly some better news related to this. There was work already going for a future release of this extension and the possibility to configure which search engine to use may be included as part of that release so soon in this case may indeed prove to be soon. Have in mind things change, but there’s a good indication that this possibility will land already in the next version (and no, don’t have a timeline for the release of the version).