I am supposed to draw the structure of a derivative of the following structure which could be used to make a protein drug conjugate. And then I should draw the reaction for the protein modification. So I thought it would be easiest to turn the carboxylic acid into an aldehyde and use this as the derivative. Then the aldehyde and primary amine on the protein could form a imine. I am not a chemist or pharmacist. So it's been a while since I had to do organic chemistry and I don't have solutions for this exercise. So does this derivative and reaction make sense?
The solution suggested by the OP might work, but has a low chance of success.
Because the drug is linked directly to the protein, it might not bind to its target. There are multiple ways this problem is solved in the literature. You could have a long linker attached to a region of the drug that is unimportant for binding to the target (you would have to be lucky, try different sites, or have more information on how the drug interacts with the target). Or you could design a linker that opens up again when the drug reaches the compartment of the target (through a pH change, a slow hydrolysis reaction, or something fancy like photoactivation).
For a super-deluxe conjugate, here is an example involving a photocaged compound: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6857671/