Is prolonging the Duha prayer without the intention of making up a missed prayer considered a bidʻah (innovation), by analogy with prolonging the night prayer (Qiyam al-Layl)?

The Duha prayer is a confirmed Sunnah. Its minimum is two rak'ahs, and its most complete form is eight rak'ahs, though increasing beyond that is permissible. Prolonging the Duha prayer by increasing the number of its rak'ahs or by extending the standing in it is not an innovation (bid'ah), as there is no upper limit to the number of its rak'ahs.