There is a biography/hagiography^(1) of Knez/Despot Stephen^(2), written by Serbian court historian, a Bulgarian called Constantine of Kostenets.^(3) I think it is a great indicator of the stance towards servitude to the Ottomans:

Before writing about Knez Stephen and Nicopolis, Constantine mentions the Battle of Rovine. In that battle, another Serb lord who became Ottoman vassal, King Mark allegedly said: I say and I pray to God that he helps the Christians, and that I be among the first to die in this war.

When mentioning Knez Stephen's role in battles of Rovine and Nicopolis, Constantine tries to be as brief as possible. In fact, Serb role in Nicopolis was not mentioned at all. That lies in stark contrast with his descriptions of battles where Knez Stephen fought the infidels: battles of Angora and Tripolje.^(4)

Knez/Despot Stephen's motivation for serving the Ottomans was described with his alleged words after the Battle of Angora in the same work: Since the [battle of] Kosovo I have been enslaved by the Ismailite people, until the emperor of Persians and Tartars has came and destroyed them and God has mercifully freed me from their hands.

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^(1) The work is a hagiography, but has so many biographical (and even geographical and ethnographical) elements that most of researchers treat it as a biography.

^(2) The title of Stephen (Stefan, Стефан) was knez until 1402. Then he received a title of despot from the Byzantine emperor. One needs to bear in mind that he ruled over his lands as knez (and in Hungarian possessions as baron), while the title of despot was purely honorific.

^(3) He was soon nicknamed Constantine the Philosopher (Konstantin Filozof, Константин Филозоф).

^(4) His heir, Lord George (Đurađ, Ђурађ), fought here with the Ottomans against him. In a similar manner, Constantine skipped his (George's) role here.