How can a marriage proposal from a maternal uncle, who is willing to cover the expenses, be refused without hurting him, especially since the refusal stems from a desire to complete studies and a feeling of not being ready for marriage?

If you can combine marriage with completing your postgraduate studies, that is best, for the Prophet, peace and blessings be upon him, said: "O young people, whoever among you can afford to marry, let him do so, for it helps him lower his gaze and guard his chastity. And whoever cannot afford it, let him fast, for it will be a shield for him." But if you cannot combine the two and you are confident that you will not fall into sin, then there is no objection to informing your uncle that you will marry his daughter after completing your postgraduate studies.