I've become efficient at doing the Diamond heist solo, in overkill 145+ and that would be a nice category to speedrun. Apart from the RNG vault the heist is not on a timer.

There are sub categories for the difficulties now. But you're not entirely correct there, there's still RNG involved on 145+: the spawn of the manager, whether you get the codes from Ralph and the timeout after escorting the CFO, which can be either 60 or 90 seconds.