Zaxx40
You are in slightly different territory right now than my and Soix experience, as your Venus 15th DAC already uses the high end OCXO (oven controlled crystal oscillator), and Fred's Terminator 15th uses dual OCXOs. So if you were to add a DDC, it would have to be the Denafrips Hermes or Gaia so you don't downgrade the clock.
Typically (though not always) the DDC sets the clock, and the DAC is the slave to that clock when using I2S. With USB and the other connections, the DAC clock is most frequently used.....in my experience (and a bunch of others), I2S is by far the superior connection. I've found the same in other systems that I've helped friends set up.....using various Denafrips DDCs. The more expensive the streamer, there is possibly less of a need for a DDC; possibly.
Part of the reason that the Pontus is improved with the Iris, as the Iris has a TCXO (temperature compensated crystal oscillator) which is an upgrade from the Pontus' standard FEMTO clock (short for Femtosecond, or one quadrillionth of a second). standard FEMTO clocks are not temperature or oven controlled, so their accuracy varies with temperature changes. The clock upgrade however is only part of the advantages of a DDC.....the Denafrips Iris ONLY has USB inputs; no AES, RCA or optical SPDIF, the Gaia and Hermes allow for those.
The other advantages of a DDC are: isolates the input signal (USB, AES/EBU, RCA or optical SPDIF), converts the signal from digital to analog, filters the signal both galvanically (magnetic) and optically, and re-clocks it with the superior crystal.
Digital devices use I2S internally to transmit data, using I2S to transmit from the DDC to the DAC allows for one less conversion.
If it were me (and I will be upgrading soon), I'd buy an Innuous/Lumin/Aurender/Grimm/etc high end dedicated streamer and see where you land. With one of these high end streamers, you may find you do not need a DDC......and you can always add one later if you wish.