Tanks sustain isn't really the core issue... It's the whole gameplay of spamming one button.

If they removed all sustain from all tanks and buffed up damage from enemies, even then it would still be just one button spam but instead of spamming Glare it'd be spamming Cure.

Some other kinds of utility, and more attacks is what would fix the problem. Heck, even something as simple as an actual positive way to engage with the Job Gauge would be a huge help... I have said it multiple times but as a WHM, I have to overheal with Lilies in order to use Afflatus Misery, which they had to buff several times in the past in order for it to not be a DPS loss compared to just spamming Glare. If you could actually spend the Lilies to do something other than waste them to get the Afflatus Misery, that'd be nice.

Sage kind of has a similar issue, they need to waste Druochole on a regular basis to regain MP to keep spamming more Dosis.