Thanks for checking back in @WarrenBelz! I didn't get the result I was looking for, but it's a technology limitation.
Ultimately I'm super disappointed that the same SharePoint form can be accessed using two links:
1. https://[tenant].sharepoint.com/sites/[SiteName]/Lists/[ListName]/EditForm.aspx
2. https://web.powerapps.com/apps/[appid]
Both links open the same SharePoint form customized with Power Apps. Here are the differences between using the two though:
Licenses
The first link can be accessed by unlicensed users, including external/guest users in your tenant. The second link can only be accessed by user with a valid Power Apps license.
Parameters
The first link can only use the parameter "ID" to pass in a valid SharePoint ID, as well as other SharePoint-official parameters. The second link can pass in ANY valid parameter that you define using the Param() function within the app.
Full-screen
The first link cannot become a full-screen experience. The closest you can get is by appending the url with "env=Embedded" to remove other SharePoint navigation, but you will always have the "Cancel" and "Save" buttons at the top, as well as the breadcrumb of "[Site Name] > [List Name] > [Item Title]" when editing an item. The second link can become full screen by using the parameter "hidenavbar=true", though you must account for ways to save/cancel from within the form, because the buttons will not be available in the header as they are in SharePoint. In both cases, you will be unable to build a truly responsive app because those settings are removed for custom SharePoint forms.
Multiple Content Types
In both cases, the app will not read a specific content type selected from the SharePoint "New Item" menu, which is a major limitation from the Power Apps technology side. It is well-documented that you will have to have a second selection within the new form to select content type, even though it was selected once from SharePoint. If you are in a scenario where you can use the second url to access your app (web.powerapps...) then you can construct buttons in your site that will pass the content type name to the app as a parameter, and avoid the extra clicks. However, if you can only use the first url (access through SharePoint), then you must have the user select the content type again within the app.
That's the essence of my findings. I recorded a video of launching a full-page experience by default from SharePoint instead of having the app appear in the sliding pane, if anyone is facing the same issue and can use the web.powerapps... type of url. That video is here: Full Screen SharePoint Forms - YouTube
