FT8 does not even require SSB as it has no amplitude information. PSK31
does for an example. The 5351 can generate the tones directly so
FT8/FT8-call and friends are more of an software Encode and Decode
Also running I2S at higher speeds is far from uncommon. Most parts easily
do that to the megabit/second level. The base spec of 400khz has not kept
up with serial EEproms and other devices where speed is a useful attribute.
As to DDS, most of them have a phase register that might be easier to
use. However it has to be one of the very high end parts to avoid problems
with SFDR. Since the 9850 is pretty poor for SFDR and still nowhere as
cheap a 5351 the choices may be a matter of balance.
The other side of the coin is expensive DDS and CPUs that can do the
RX side as well as TX pushes for STN32F4 class parts and full out SDR
which avoids some of the reconstruction problems and possible errors.