Unlike multiplayer missions, where players can adjust their desired parameters in the Respawn Window, single-player missions—especially those with custom content—do not offer this option.
However, the unitSetProperties trigger can be used.
Specifically, the bombDelayExplosion option (found in the Attack params subcategory) allows setting a fuse delay in seconds for either the player's unit or an AI unit.
Interestingly, this option provides much finer control over the fuse timer—down to fractions of a second or even beyond 30 seconds.
That's basically it.