The WordPress login page is not just for Admin users. Any user with an account will be able to login through it. There are lots of sites out there which uses the default login page for their site’s login with a new theme.
With that said I’m not asking you to change your site’s login page to default page. That was for testing and debugging purpose only. Just like I asked you to deactivate other plugins and change your theme. I’m not asking you to permanently deactivate all your plugins. All these actions are to find from where the issue is coming from. We can only safely fix an issue, if we know from where it’s coming from.
Since the redirect works on the default login, now we know the login redirect configuration works. So we can eliminate that variable. The issue seems to be your font-end login form discarding the redirect configurations. You mentioned that it’s hard coded there. Can you send me the code?