For what it’s worth, it’s possible use Rec. 2020 without any HDR. There is a benefit to it, albeit small. Some commercial discs do it, and (UHD) player compatibility is not an issue
Or the better route, in my opinion, would be to perform all restoration and grading work within Rec.2020 ST2084, but with an output transform set to Rec.709 Gamma 2.4. So effectively, it would be as if you were making the restoration in Rec.709; however, all of the hard restoration work would be preserved in a larger container, thus future-proofing the restoration.
In other words, you’d still view everything in Rec.709, and export everything in Rec.709, and release it in Rec.709. But internally, you can always disable the output transform and you’d have all the extra detail there. That way, in 5+ years from now when HDR grading becomes much simpler, none of the restoration work has to be redone. The only thing it will need is a new color grade. 🙂
And you can do all this with a standard SDR monitor.