I am sure you can go 5.4 to 7.1, but not to 7.5, it would have to be a 2 step migration.
You will also need to look what hardware you will migrating to, 7.5 will only run on Power9 and Power10, and there is only 2 or 3 Power9 models that will run 7.1. There will also be changes to the OS and services, that might have an impact, e.g. if if you use SFTP the client and server have big changes to the ciphers, with IBM dropping any it regards as week. I found this going from 7.2 to 7.4, and that was only a little jump, expect some others.
You will also need to check any third party products and will they still run/be supported/upgradable, and any potential addition licencing costs.
If I was you I would get a TA in to look at at it, I know it is not cheap, but at the end of it, you may be very glad you did.