Is it considered a dereliction of grace and a sin to abandon a job you desired and acquired, only to regret it later? And is it considered an تجاوز (transgression/overstepping of bounds) to ask Allah for forgiveness and provision, either with another job or ample provision for a private project?

Everything that befalls a servant, whether good or evil, is by the decree of Allah. A believer must be content with Allah's decree, thankful for His blessings, and patient with His trials. The human intellect is limited in comprehending the true nature of affairs, so submitting to Allah's decree is better for one; for good may lie in what was withheld, and evil in what was desired. Do not grieve over what you have missed, and there is no sin upon you for leaving the job. However, learn from the experience by not rushing into decisions and by exercising patience and wisdom in addressing problems. Abandon regret over what has passed, and seek what benefits you in your religious and worldly life. The door of Allah is open and His mercy is vast, so strive in supplication and persist in imploring Allah Almighty to facilitate goodness for you. Your supplication is not a boast but an act that Allah loves and rewards its doer. It is recommended to observe the legal Islamic rulings in the chosen work, and your decision to stay at home is best unless there is an urgent need for work.