I also own the new PSA SACD Transport for almost a year now. I have it paired with a PSA Gain Cell DAC, as I could not afford both the new Direct Stream and the Transport at the same time.
To get all the SACD goodies that the PSA has built into its SACD Transport, you will need to use a current model PSA DAC. Period, end of story.
Does not matter if a competitor's DAC has an I2S input, it needs to be a PSA DAC. I initially paired the Transport with a GUSTARD DAC, as it had I2S connections, but it doesn't matter. Needs to be a PSA DAC. With the Gustard, I had to connect to the transport separately for CD Redbook and SACD, so I had to use both I2S and something else.
I purchased a used version of the Direct Stream from 9 years back. GREAT DAC in its own right, but the new Transport does not support older PSA DACs. End of life situation from a software perspective.
PSA was great about it. Paul worked with me and gave me credit for the used DAC that I could apply to the Gain Cell. So net net I was ok and could use the Transport as intended. Keep in mind though, the Direct Stream is in a different league than the Gain Cell.
Check with PSA directly and see if a used Direct stream would work out and what version it would need to be.
Good luck,
Rich