I recommend the following books all of which are very good.
1. (1984) 2021. Ebbinghaus, H.-D., Flum, J. and Thomas, W. Mathematical Logic. 3rd ed. Springer.
2. (2006) 2010. Rautenberg,W. A Concise Introduction to Mathematical Logic, 3rd ed. Springer.
3. 1967. Shoenfield, J. R. Mathematical Logic. Addison-Wesley Publishing Company.
4. (1972) 2001. Enderton, H. A Mathematical Introduction to Logic. 2nd ed. A Harcourt Science and Technology Company.
5. 1976. Monk, J. D. Mathematical Logic. Springer.
6. (1995) 2013. Hodel, R. E. An Introduction to Mathematical Logic. Dover Publications, Inc.
7. 2005. Hinman, P. G. Fundamentals of Mathematical Logic. A K Peters.
8. 2023. Avigad, J. Mathematical Logic and Computation. Cambridge University Press.
9. 2023. Mileti, J. Modern Mathematical Logic. Cambridge University Press.
10. 2024. Open Logic Project. The Open Logic Text. Complete Build. 2024-12-01.
For difficulty of them, I list the ten books as follows: 1 < 2, 3, 4, 5, 6, 8, 9, 10 < 7. Maybe the difficulty of 2, 3, 4, 5, 6, 8, 9, 10 could be distinguished furthermore, but for me they are almost the same. By the way, I've ever investigated almost all of the books on mathematical logic or symbol logic, and give a table to show indicators of many aspects of about 30 good books, while unfortunately I write it into my book in Chinese of which the English version may be published very late.
Furthermore, since you are a graduate in the department of mathematics, the books numbered as 2-7 and 9 are very suitable to you, but 2, 4 and 9 are better as introductory books which are very modern, and the left could be further readings.