A proxy account is a normal domain account - it is just setup with a strong password that never expires. It is not setup as a normal user and doesn't need any permissions other than those explicitly defined for access to the database and shares.
To create a proxy - you add a credential in SQL Server - that credential is where you put the domain account (e.g. domain\svc-account) and password.
Once you have a credential you then create a proxy in the agent - using the credential. You then add that proxy account to the Integration Services subsystem.
The agent job step can then be modified to use the proxy account when that step executes instead of the default SQL Server Agent service account.
I recommend you do some reading up on creating proxy accounts - they are not difficult to setup and use and are the recommended method within SSIS.
Finally - changing the account running the service back to NT SERVICE\SQLSERVERAGENT does not require entering a password. This is a special account only available on that system and does not have a password. Using a gMSA account also does not require entering a password because the authentication is handled by the domain.
Instead of just brushing off these ideas as being too complicated - do a bit of research.