I really think it depends on where you are. As Rice mentioned, DRM is an issue, but you also find out which stations broadcast in HDR and Dolby Atmos, because that makes a noticeable difference.
I have two HDHR Extends for Channels DVR, but I also have a ZapperBox that handles DRM. In the SF Bay Area, FOX, NBC, and CBS are DRM, and ABC and CW do not have DRM and can be tuned by the Flex 4K. Only CW and NBC broadcast in HDR and Dolby Atmos, and these are the only two stations that I think look noticeably better than their ATSC 1.0 counterparts. FOX, CBS, and ABC look pretty similar in both ATSC 1.0 and ATSC 3.0, IMO. So for me, it is not worth it to get a Flex 4K just to record CW with better picture and sound quality. I do occasionally watch/record NBC using the ZapperBox, but I use Channels DVR and the HDHR Extends for almost all my OTA recording/viewing.
If you have any stations that you enjoy watching that are not DRM and broadcast in HDR and Dolby Atmos, you would might enjoy having a Flex 4K. Otherwise, I would wait. It's possible that we might see the FTC allow stations to start broadcasting ATSC 1.0 using MPEG-4, which might result in better picture quality with your existing tuners. At least that is the prediction at the end of this excellent recent video about how the ATSC 3.0 transition is going.