If you don’t use the custom login lightboxes then you can either use the default Wix signup form or the custom signup form through Wix Forms.
https://support.wix.com/en/article/about-login-signup-and-password-windows
https://support.wix.com/en/article/about-the-member-signup-form
If you use the default Wix signup or the custom signup through Wix Forms one, then you can simply have the default option in the settings to have the ‘Join the Community’ checkbox pre-ticked, so that new users who signup will automatically be members of the Wix Blog and Wix Forum.
https://support.wix.com/en/article/editing-your-member-signup-settings-for-the-default-form
Join the community is checked by default: Click the toggle to check or uncheck the “Join the community” checkbox by default.
Note: This option is only relevant if you have a social app such as the Wix Blog or Wix Forum on your site. When the toggle is on, members are added to the community by default.
Learn more about Wix Blog and Wix Forum communities.
I myself use both my custom login and signup lightboxes along with the Wix Members app itself.
The only difference being that my Member Profile pages are not included in the Wix Members app pages in the site structure, they must be setup as their own separate dynamic pages.
The rest of the Wix Members app pages that I use are the My Account page, profile page, all the blog and forum pages, notifications, settings etc. If you use Wix Stores too, then you have extra pages like Addresses and Cards in that Members app menu too.
https://support.wix.com/en/site-members/setting-up-a-members-area
With the pages that are shown in the Wix Members app menu, these can be shown or not shown in the menu for the app.
https://support.wix.com/en/article/hiding-a-page-from-the-member-menu
As for the Login Bar that comes as an additional feature of the Wix Members app, I do not use this at all.
You can still use it yourself and edit it to suit yourself and your website.
https://support.wix.com/en/article/hiding-a-page-in-the-member-login-menu
https://support.wix.com/en/article/hiding-a-page-in-the-member-login-menu
However, as it is a part of the Wix Members app, you can’t get to edit it at all through any use of code, hence why I do not use it on my website at all.
As for the Login Button, I only have it shown on my Members page which changes to logout after members have logged in, this is the only place that members can login and logout of my site, kept it simple and all in one place.
It should work perfectly fine if you place a Login button in your header, the only difference being is that your button will now be working site wide and so your code for the Login will have to be in the site code tab as it will need to apply to your whole site, instead of just being on the one page like I currently have it as it only applies to that one page.
Finally, you only need to refresh the page that is shown underneath the lightbox if you are wanting to stay on that same page.
If you are moving the user to another page, then you simply replace the refresh page url with the url of the page that you want users to be moved too.