Is it better in daytime supererogatory prayers to combine prolonged standing (qiyam) with frequent prostration (sujud), based on the hadith, "Whoever bows a bowing (rak'ah) or prostrates a prostration (sajdah), Allah will raise him one degree thereby and remove one sin from him thereby"?

Combining a lengthy standing (qiyam) with frequent bowing (ruku') and prostration (sujud) is best if there is sufficient time. As for the story of Abu Dharr (may Allah be pleased with him) that the questioner referred to, it is narrated in Musnad al-Imam Ahmad, Ibn Abi Shaybah, al-Bayhaqi, and al-Tahawi. The correct wording in it is "He does not lengthen the standing but performs many bowings and prostrations." This is supported by the narrations of al-Tahawi, al-Bayhaqi, and Ibn Abi Shaybah, as well as the statements of hadith commentators, and the fact that the story is often cited in the context of preferring one over the other.