Do the preacher's feelings of happiness due to a large number of likes and comments, and depression due to their scarcity, as well as his intention to earn money through lawful means from his da'wah activities, invalidate his work and make him lose sincerity to Allah Almighty?

Teaching people and disseminating religious knowledge is a great door to good deeds, and a Muslim should adhere to it and not abandon it due to misgivings.

Sincerity to Allah alone is a prerequisite for da'wah (inviting to Islam) and teaching people. It is not permissible for a Muslim to intend by his work the praise of people or the acquisition of worldly gains, because this compromises sincerity and falls under minor shirk (ostentation).

Rejoicing in people's praise is not considered ostentation if it was not intended, and its reason was the love of good and the people benefiting from the da'wah. However, if its reason was the love of praise and fame, then it is forbidden ostentation.

Earning money from da'wah work does not diminish sincerity as long as it is not the primary motive for the work. It is permissible to spend it on developing da'wah activities or to use it to dedicate oneself to da'wah.

The criterion for knowing what compromises sincerity is the servant's intention and purpose. If the motive for the action is sincerity to Allah and hoping for His reward, then he is sincere.