Honestly this just shows us how important it is to have a decent progression of abilities while levelling. BLM got fixes to its rotation in Stormblood that also helped its levelling period massively. SCH had its issues addressed by the role action system. There's no reason SE can't implement similar ideas on other jobs. Every Tank except WAR has both of their stances and gets their basic aggro abilities at similar levels before reaching level 50.

On Machinist you could easily make Gauss Barrel a flat damage bonus trait at 52, then upgrade it to unlock the Overheat mechanic when you unlock Cooldown at 62.

With AST swap Gravity around either Combust II or Synastry. Fixes their level 50 sync issues and changes very little of of their progression.