The best way I have found so far is a combination of https://github.com/ViGEm/ViGEmBus and https://github.com/Snoothy/UCR. You install the ViGemBus driver, then use UCR to map all the inputs of the DualSense to DualShock 4 outputs. I then hid the standard "wireless controller" Steam recognized, and then I had a fully recognized DualShock 4 controller. There's sadly no way to use Gyro until Steam adds official support for the controller, but this works for the normal controls. When mapping the triggers, I had to program them as "Button to axis" as I was having an issue with it holding down when it's not supposed to. This means you can't use the variable trigger press, so you can't do that thing where you move the hammer but not shoot hand cannons, but that's probably just a bug I haven't been able to get around yet.

Thanks for the response! I might try those programs. I have been able to get the DualSense to work in Steam. I first had to configure each button under the General Controller Settings (it was registered as a PS4 Controller) then configure the button layout for Destiny (using their recommended layout). All the buttons work as they should. No rumble (really want this, especially for bows!). I can't recall if the triggers ended up being axis or single clicks (your hammer on the handcannon example). I'll have to check later. Not sure how to contact Steam on the DualSense support, I can't find a category for hardware use that allows me to contact support directly with this issue. Would be great to get an ETA on when this controller will be more supported (particularly the new haptics).

I contacted Steam support, and they said official support would be added sometime after the PS5 launch.