It was certainly the product of some experimentation over time. Displayport method did not work for me. I'm using a HDMI 2.1 cable, I couldn't tell you the brand offhand. From a generic big box store.
Go to the Extensions section (button near bottom right). You'll end up having to click to add a Freesync range block, then set it. I'd recommend 47-61 actually. 45 was a bit arbitrary, and after testing a bit more, I do see some frame drops in 45-46. 62 causes noticeable drops on mine. Save/apply and exit.

After that, go to AMD Display Settings and you should be able to enable VRR if the last step took. I'm using an RX 6400 here. Then you'll need to there create a custom resolution of the native res (2560x2880) at 61hz.
Next, go to Windows Display Settings, then Advanced Display Settings, then Display Adapter Properties to manually change the desktop refresh to 61hz. Do not skip this step! Calamity actually posted about this a while back, that VRR apparently has a tolerance range from the set target where it does not kick in. Net effect is, if it's still set to 60hz, things close to 60 (CPS, for example) do not sync right. I've seen that behavior on mine. Lifting it to 61hz apparently 'tricks' VRR into turning on as intended at those between 59-60.
At that point, you can go verify it's working with various freely available tools. The scrolling bars VRR test on git, the pendulum test, Blurbusters frame drop test, whatever floats your boat. Mine appears to be working spot on within the set range.
Last, tweak MAME ini as needed... almost stock will do. Turn lowlatency on (1), and make sure refreshspeed, syncrefresh, and waitvsync are all off (0). And... that's about it, should work fine. Turn off filter and go nuts with HLSL.
Let me know how it turns out... Excited to see someone else appreciating one of these, and hoping mine's not a fluke.